Sequence of Events - Alison's Disappearance

Day of Alison's Disappearance- Chronology of Events on September 1, 2009

Morning
Alison (presumingly dressed as Vivian Darkbloom) calls Duncan Albert to come pick her up in his airplane from Hilton Head. He drops her off in a field nearby Philadelphia, concluding that Alison was secretly in town 6–7 hours before the girls believed her to have arrived. (Airplane scene between Aria and Duncan, Eye of the Beholder)

Alison checks into the Lost Woods Resort under the alias "Vivian Darkbloom". She has also checked in once before. She checks into Room 1. (The guestbooks found by Spencer and Aria, UnmAsked)

Sometime after it is assumed Alison, after a long correspondence through personal ads in the The Rosewood Observer, goes to finally meet/confront "A" in front of the creepy Doll Hospital in Brookhaven. It is unknown at this time how the meeting went, or if they met at all. All that is known is that the meeting was scheduled. (Spencer's discovery in newspapers, If These Dolls Could Talk)

Afternoon
Alison visits Jenna at the hospital, shows Jenna a video of Jenna and Toby together, and blackmails Jenna into keeping "The Jenna Thing" a secret. Alison tells Jenna that the video will be kept "under lock and key." (Flashback, For Whom the Bell Tolls)

Sometime before visiting Emily later in the day (presumably on her way back home), she bought a storage locker, into which she deposited a flash drive containing Ian Thomas's creepy/pervy videos, inside one of her old lunch boxes.

Alison returns home, pulling up in a taxi, and is greeted by the girls after "supposedly" returning by bus from her summer vacation at her grandma's house in Georgia. She is beautifully tanned, but also complains that her arms are sore. Spencer sees a Hilton Head resort tag on Alison's luggage. After asking Alison about Hilton Head being in South Carolina, Alison responds, "Why so many questions?" Alison seems excited, saying she can't spill every detail and to "wait for it, wait for it..." (Flashback, Salt Meets Wound)

Alison then visits Emily and gives her a snow globe, in which a key to said storage locker (with the label) is hidden inside. Alison tells Emily to keep it safe because it's "a lot more valuable than it looks." Alison tells Emily she needs to go because she has "a prior engagement" but that she will meet up with the girls later. (Flashback, Monsters in the End)

Alison then leaves Emily's house, claiming she has a "prior engagement."

Ali has dinner with her family before the sleepover. (Mentioned by Aria in Never Letting Go.)

Evening
Alison arrives at Spencer's barn, purposefully scaring the other four girls in process, before proceeding to hang out at their sleepover party. Sometime later in the night, the girls, except presumably Alison and Spencer, all fall asleep drunk. (Scenes, Pilot)

Alison and Spencer go into the house and argue. Alison pressures Spencer to tell Melissa about Spencer and Ian, or she will. Spencer says to Alison that she is sick of her games. Spencer says, "You are dead to me already." Someone takes a picture of Alison in front of Spencer's barn, with Spencer's shadow in the shot. (Flashback, If At First You Don't Succeed, Lie, Lie Again)

Alison meets up with Toby. According to Toby, he and Alison met outside of Spencer's barn, and Alison borrowed his sweater because it was cold. When they part, Toby sees Alison get into a car with an unidentified man (possibly Ian), and she is still wearing Toby's sweater. It's not fake because Marlene tweeted that Toby was the only honest person in Season 1. (Scene, Keep Your Friends Close, Toby to Emily).

Alison meets with Ian at The Kissing Rock, where they make a makeout-tape. Alison teases Ian, "I know you want to kiss me." Ian replies, "Come closer." A struggle appears to occur, and the camera falls to the ground, losing focus on the other two. Then, Alison's hand falls into view of the camera; her hand grabs onto some dirt and leaves, before going limp. (Scene with video that "A" sent, Know Your Frenemies) Then, in The Devil You Know, it's discovered that few seconds after Ali's hand had gone limp, she had moved it again, got up, and thanked Ian for meeting her there (meaning that Ian had not killed her in that moment).

Sometime, presumably after Alison and Ian's encounter at the Kissing Rock, Ian arrives at Ali's house before Ali can get home, and he meets up with Garrett Reynolds and Jenna Marshall who are gathered in Alison's bedroom and looking for N.A.T. Club videos that she had stolen from them (Blond Leading the Blind). While searching, Melissa Hastings arrives enraged and looking for Alison.

Garrett and Jenna leave Ian and Melissa alone, and exit the house into the DiLaurentis backyard. Alison confronts them, and reminds Jenna of the promise she made if she ever came back to Rosewood. Jenna tries to attack Alison, and a small fight breaks out. Alison pushes Jenna who falls over a lawn chair. Jenna grabs a nearby Field Hockey Stick that belonged to Spencer, and goes for Alison.

Garrett grabs the stick from Jenna. He raises his eyebrows in confirmation to Alison, who slightly nods and he proceeds to pretend killing Alison with the field hockey stick, but was really hitting the tree. Jenna asks if she's dead, and Garrett says he took care of it the two then run away.

Aria, Hanna, and Emily wake up to find Alison and Spencer gone. Spencer then appears and says she has looked everywhere but cannot find Alison and that she thinks she heard a scream. (Scene, Pilot)

A little bit later, Garrett comes back to see if Alison is okay, and is surprised to see her talking to Byron Montgomery, Aria's father. Alison says "If you don't pay for your mistakes, how do you become a better person"? Byron replies back "You say all these grown-up things, yet you're still a child". Alison smirks and says "You know what I'm capable of". Garrett makes a noise and the conversation is interrupted. He runs off leaving Alison and Byron looking on to see who was there. (Flashbacks, This Is A Dark Ride)

After Garrett left the scene, Byron continues their conversation, telling Ali that he didn't bring any money. Alison threatens Byron, and he responds by bringing up that if Alison tells Ella it will really hurt Aria, and he doesn't believe she is that horrible of a person. Alison retorts by saying that he clearly doesn't know her that well then.

Byron begins to walk away, and Alison threatens him again saying it's his last chance to save himself. Byron turns while he is leaving and replies, "Yes it is." presumably meaning he was going to tell Ella himself. Alison screams back "You made your bed Mr. Montgomery," Byron turns back and sees Melissa Hastings coming out from the DiLaurentis's back door on the phone with someone.

She seems distressed, and says into the phone "What do I have to do, call 911 to get your attention?" (talking to Detective Wilden? or Garrett?) Byron takes notice, then turns and leaves. (Flashback, Misery Loves Company)

Alison calls CeCe Drake, freaking out because Ian, Garrett, Jenna, and especially Melissa are after her, wanting the stolen N.A.T. Club videos back. Presumably after Melissa walked out on the DiLaurentis porch, she sees CeCe just arriving and talks to her, begging her to talk to Alison. (CeCe told Emily, Hot Water)

Presumably shortly after CeCe arrived, Jason DiLaurentis stumbles out onto his porch, drunk and high. After stumbling down the steps and attempting to drink from a garden hose, he sees Melissa and Cece talking about something seemingly important (CeCe talking to Ali about the videos), and CeCe is seen wearing a similar outfit to the one Alison was wearing at the time (yellow top, dark skinny jeans).

After witnessing this, Jason collapses in a lawn chair and passes out again. Jason was heavily drunk and high during this flashback, so it may be unreliable. (Flashback, What Becomes of The Broken Hearted)

CeCe finds Ali, and talks to her. Alison says she's going to go get  the videos, but never returns. (CeCe told Emily, Hot Water)

At the end of the night Alison's murder takes place. She is hit in the back of the head with a shovel possibly in an ambush or attempt to get away from her killer, and then buried alive.

Her hand is seen poking out of the ground attempting to get out. Someone comes and pulls Alison's hand out of the ground, revealing Alison might have survived. However, it has yet to be comfirmed wether or not Alison is actually  alive or dead is not yet confirmed. (Flashbacks, This Is A Dark Ride and A DAngerous GAme )