Speculation: A man with a hooked nose shouting at a cowering woman while a dark-haired boy (Severus since it's his memory) crying in a corner.

Student at Hogwarts

Snape enters Hogwarts on September 1st, 1970.

He is sorted into Slytherin House.
Professor Horace Slughorn is the Head of House from 1938 to 1980.
The Marauders (James, Sirius, Lupin and Pettigrew) and Lily Evans enter Hogwarts
Lucius Malfoy is entering his 6th year.

"Snape knew more curses when he arrived at school than half the kids in seventh year, and he was part of a gang of Slytherins who nearly all turned out to be Death Eaters." (Sirius)

Riddle resurfaces, calling himself "Lord Voldemort," and begins his quest for power and immortality. Most say he started in 1970. If so, then he kept quiet enough until James Potter's 5th Year because he wished Grindelwald back since there were no other dark wizards around.

 Fifth Year, 16 years  and 5 months old: Snape sits his O.W.L.s and is humiliated in front of the whole student body by the Marauders after (Snape's"worst memory).  

• Expelliarmus used by Potter (Snape loses his wand)
• Impedimenta used by Sirius to knock him off his feet
• Called names (Snivellus) and insulted about his greasy skin and nose by Potter and Sirius
• Scourgify used by Potter to wash his mouth which filled with bubbles, he was gagging and choking
• Lily Evans comes to the rescue
• Snape finally reached his wand: " a flash of light and a gash appeared on the side of James's face, spattering his robes with blood" (May be RectusSempra)
• Humiliated through public display of his underpants by Potter: "A second flash of light later, Snape was hanging upside-down in the air, his robes falling over his head to reveal skinny, pallid legs and a pair of greying underpants."
• Potter lets him drop on the ground and petrifies him.
• Potter undoes the curse thanks to Evans, but Snape calls her a mudblood (doesn't want her help)
• Lily goes away angry at both Potter and Snape.
• Potter hangs Snape upside-down in the air again and asks if someone wants him to take off his underpants. Whether this happened or not is unknown.

Sixth Year: (from the HP-Lexicon) " Sirius tricks Snape into following Lupin through the passage beneath the Whomping Willow. James Potter, realizing that Snape would encounter a full-blown and very dangerous werewolf at the end of the passage, stops Snape from going through, thereby saving his life. Snape, much to his disgust, is indebted to James Potter because of this. Dumbledore calls this a "life debt.""

Coming of Age

Snape is in his 6th Year at Hogwarts when he comes of age on January 9th, 1976.

It is unknown if he took the Dark Mark after his coming of age or before. Though logically, such magic could prove detectable before a wizard is an adult.

Graduation

Snape graduates in June 1977.  He is 18 years old.

From June 1977 to Fall of 1979/June 1980 (prophesy unveiled)  his whereabouts are unknown.  

In between the Fall of 1979 up to June 1980

Pettigrew starts passing up information to Voldemort in 1979.

On a "cold, wet night", The Prophecy is revealed by Sybill Trelawney while Snape eavesdrops. He hears the first part of a prophecy about a boy to be born at the end of July (Harry or Neville) who would have the power to defeat the Dark Lord.

Snape was surely already a Death Eater because Sibyll mentioned he must have wanted some tips for his own interview. Also, knowing that back then the Order members were outnumbered twenty to one, we can assume he was.

Snape was seeking a job at Hogwarts upon the orders of Voldemort. If Voldemort ordered him to do that, then it would prove he was a Death Eater when he heard the prophesy.

Spy Days

Summer 1980 up to the Dark Lord's fall, Halloween 1981

(spied for at least 13 and a maximum of 25months)

Snape had been a Death Eater for an unknown period. Possibly from his coming of age up to sometime after summer 1980 and before the Dark Lord's fall in 1981.

Snape had become a Death Eater (for sure) by the time he came back to spill everything out to Dumbledore. He wore the Dark Mark:

Every Death Eater has the sign burned into him by the Dark Lord.
It is branded on the left forearm.
It is a means of distinguishing one another
A means of summoning us to him. When he touches the Mark of any Death Eater, their scars burns and they are to Disapparate, and Apparate, instantly, at his side.
The Mark burnt dark when Voldemort arose a second time and lost its dark shade after less than an hour.

Dumbledore said Snape rejoined their side and turned a spy at great personal risks before Voldemort's downfall . So he'd have time to spy on the Death Eaters. Therefore, he went to Dumbledore long enough before Voldemort's downfall.

Snape turns a spy after going to Dumbledore and when he understood what a terrible mistake it had been to inform Voldemort of the half-prophesy because it endangered the Potters and their baby(Harry).  Dumbledore thinks Snape's worst fear was when he realized that Voldemort decided to go after the Potters because of the Prophecy (HBP25). However, when Voldemort declared that he would kill the parents and the son himself is unknown. It may be right after his learning the Prophesy or up until October 24th 1981 when the Potters went into hiding using a Fidelius Charm. Since Albus said Snape had turned back to the Llight before his downfall, it is safe to presume it was not only a week prior as if to save his own skin. Since the charm was cast more than a year after the prophesy, it is believed Voldemort did not express his wish to kill the Potters right away or surely Snape (if he was that repentant) would have told them to do the Fidelius Charm before. When Snape felt guilty, he may have overheard Voldemort's plans or been told directly and he ran to Dumbledore.  Unfortunately, we do not know when he did realise they were in real danger but I believe Snape's wise enough to understand that being an enemy of the Dark Lord is already dangerous, so he may have done so pretty early after telling him. Especially since he hired in for September 1981.

Inference: Snape gave Dumbledore an excellent proof of his faithfulness since Albus' trust in Snape is unshakable. Especially if he offered him a teaching position. Speculation has it that they performed an Unbreakable Vow.

Begins teaching at Hogwarts

He was interviewed before September 1st, 1981 when he was offered a job. It took him more than 12 months to get it even though Voldemort had ordered him before Sibyll got hired. He was supposedly a spy for Albus by that time and that is why Albus gave him the job, especially after Slughorn retired to go into hiding himself. Coincidence? That is how Snape became Head of House.  However his real allegiance may have been to either master or none.

• Professor at Hogwarts' school of Witchcrafts and Wizardry
• Official position : Potion Master (American version: Potions Master)
• The Defence Against the Dark Arts position was refused to him. He tried each year until September 1996 when he finally got it.
• Head of House: Slytherin
• Snape had left Hogwarts in June 1977 and came back on Sept. 1981. He was absent for 4 years. Hence some sixth or seventh years may have known him as a student.

Downfall of Voldemort

Snape was not mentioned as having participated in the Fidelius Charm. Therefore he was not supposed to know where the Potters were hiding. However, since Wormtail betrayed his friends, the Dark Lord may have told Snape of his plans to kill them. It is hypothesized that Snape was there on that night or not but asked the favour of not killing Lily. But it could be anyone else who made that request because it was mentioned he had given her a chance to step aside.  

Harry 'killed' Voldemort on Halloween 1981. Snape had been teaching for 2 months.

Trial

Dates unknown. Surely within the year after Halloween 1981.

On November 1st, Sirius Black is condemned to Azkaban without a trial. Rowling said he was about 22 when he was imprisoned.

Snape stood trial before Karkaroff.

Albus defended Snape at his trial and vouched for him. He also did it again when Karkaroff declared he had been a Death Eater as well.  

Teaching Years

Always was the Head of House of Slytherin
Taught Potions for 15 years (from Sept. 1981 to June 1996)
Taught DADA for 1 year (from Sept. 1996 to June 1997)

It says in the HP Lexicon that he started in Sept. 1981 at Hogwarts while Trelawney started a year before in 1980. In Order of the Phoenix (November), Trelawney said she had been teaching for almost 16 years, and Snape 14 . So why the two year difference between those two teachers?  I reckon that Snape said 14 full years when Umbridge asked him in Book5. And Trelawney said almost sixteen though in fact, she was merely starting her 16th year.  

By the end of Book6,  he had been 16 years of teaching.

No known crucial events before Harry Potter's arrival at the school

Harry Potter
Years 1 to 5

1st Year: finds out Quirell is the villain. Refers a Quidditch match to protect Harry. Composes a potion riddle to protect the Philosopher's stone, protects Harry all year.

2nd Year: catches the boys who took the Ford Angelina, secretly investigates the weird incidents in the school, duels with Lockhart on December 17 (must have been his Xmas present ; )

3rd Year: taunts Lupin, Neville's boggart, teaches at least on DADA class, catches Potter out at night with the Marauder's map, investigate about Black and Lupin secretly, and in the end hunts him down to protect the kids and to seek a kind of revenge for himself (and the Potters). He's furious when he escapes and blames Potter.

4th Year: The Dark Mark gets clearer all year long, Snape avoids Karkaroff, Snape secretly keeps an eye open on Potter because of the attack at the Quidditch cup, Moody annoys and spys on Snape, Severus reveals his Dark Mark to Fudge, and becomes a spy again. He meets Sirius and they call off a fragile truce (not insult-free mind you!) He left 2 hours after the Mark burnt black, therefore right after he left Albus in the hospital wing.

5th Year: Snape reports to the Order about his spying, he is now a member of the Order, Umbridge inspects his classroom on October 7th and mentions he's been teaching for 14 years, announces the Occlumency lessons on January 11th, he gives Potter some Occlumency lessons starting Jan. 13th, the animosity between Sirius and Snape is obvious, gives Umbridge false Veritaserum, Harry ends up discovering three memories of Snape and his worst one (start of April), too. Snape refuses to acknowledge Potter again until he tells him that Sirius is in trouble. Snape does all he can to help Potter and Sirius at the end of the book without blowing his cover. HP-Lexicon: "Snape saves Harry’s (and many other lives) by alerting Dumbledore when he found out that Harry had gone to the Department of Mysteries. However, Snape does not participate in the battle. "

Harry Potter 
Year 6

Summer of 1996: Albus comes back to Hogwarts in a bad shape, Snape saves his life. He is revealed as one of the most praised DE of Voldemort. Narcissa makes an Unbreakable Vow with Snape to protect her son no matter what and to kill Dumbledore if he fails. Snape inhabits Spinner's End where Wormtail is also residing, supposedly helping him.

Always requested the DADA position but was only indulged in 1996-97
Taught DADA for one year

Snape tries to 'help' Draco but he won't. Snape is unaware of Draco's actions the day he ends up killing Albus. Snape flees and reveals himself as the Half-Blood Prince. He also makes sure Potter is intact and even gives him last-minute tips on how to vanquish his enemies.

End of school year 1997 - In hiding. Fled from the school after killing the Headmaster.

• Before July 7th - Bella and Narcissa go to see Snape; Snape makes Unbreakable Vow
• September 1st - Tonks rescues Harry, gets insulted by Snape who takes him to the welcoming feast
• Sept. 2nd - DADA class: Snape tries to jinx Harry, Harry retaliates and gets detention. Harry gets HBP's book (Snape forgot it behind)
• Sept. 14th - Harry serves detention with Snape - Flobberworms
  
• Dec. 20th - Malfoy caught in corridors, Snape takes him to classroom where Harry overhears Snape trying to help Malfoy
  
• Dec. 24th - Harry talks with Lupin and Arthur about Snape, Greyback
  
• Jan. 5th - Harry talks with Hermione about Snape
  
• March 1st - Snape argues with Dumbledore
• May - Harry finds Malfoy in bathroom, uses Sectumsempra on him, gets detention from Snape for rest of year
  
• May 29th - Trelawney reveals that Snape overheard prophecy, Harry and DD return, Dumbledore confronted by Draco, Snape kills him. Harry chases Snape off grounds
  

Waiting exasperatingly for Book7 or some information by Rowling!! x_x