What are the most important events throughout the book We Were Liars, written by E. Lockhart?

The important events are not all revealed in chronological order, as much of the novel concerns Cadence Eastman's amnesia and memory recovery process. The final full recovery becomes a significant event near the end
Beginning with her fifteenth summer, Cadence Eastman has numerous important experiences that constitute the main action of We Were Liars. One major change that summer in Maine is her involvement with a boy, Gat. Together with Cady’s cousins, the foursome becomes inseparable. The most significant event occurs that summer with the Liars group while they are goofing around in the water. Cady hits her head, which turns out to be a serious injury. She deals with the pain in part by using Percoset, to which she becomes addicted, and she also suffers from memory loss.
The next summer brings one more important event: rather than go to Maine, she spends it in Europe with her father. At age 17, she returns to Maine. Part of the significance is what happened while she was away, especially her grandfather’s aging and related health problems with dementia. Trying to reconnect with her Liar group is unsuccessful; they avoid her. Collectively upset about radical remodeling that was done to her grandfather’s estate, they go off to a nearby camp to get away from it.
Cady’s memories finally return. She realizes that two years before, she and the group had set fire to the estate as a way to prevent its possible sale. In doing so, they had also killed the dogs trapped inside. What is worse, the other three teenagers died in the fire. Cady has been hallucinating them. Tempted to take her own life, she stops herself. The return of her memory allows her to begin true grieving and recovery.