Here's how I think it went down.
Since we suspect that she was already setting up a story on the 17th by visiting the Sawgrass as "Zenaida," then I believe that it happened on the 16th. Maybe the morning of the 17th.
I'm guessing that there was a throw-down between Cindy and Casey on the night of the 15th/morning of the 16th about Casey's lifestyle...Cindy was always having to watch Caylee while Casey partied like a rock star and refused to get a job. Additionally, Casey always asserted herself as Caylee's mother, but took no responsibility for Caylee--her hackles went up anytime Cindy said something about the party environment that she exposed Caylee to, and about Casey doing nothing to help with household bills. But mostly, she wanted Casey to be a responsible parent to her daughter, and to realize that mothers don't go out and party and sleep around when they have children...but Casey laid the guilt trip on Cindy..."You told me when I came to you pregnant that you wanted me to have this baby. But I never intended to give up my social life!" Casey got sick of telling Cindy, "I'm her mother, so you really have no say in how I raise Caylee. If you don't like it, we'll just leave, and you'll never see her again."
Casey and Caylee left.
In anger and because of resentment toward Cindy, Casey retalliated against Cindy the most painful way she could. She killed Casey on the 16th not long after a verbal spat with her mother...and then she set in motion actions covering the murder with the babysitter story.
She visited the Sawgrass as "Zaneida," to set up the babysitter story. A week later, though, she still had Caylee's body in the trunk. She went home on the 22nd or 23rd to bury the baby in the back yard. She backed into the garage, then went next door to borrow the shovel. She unloaded Caylee from the trunk, and carried her and the shovel to the back yard. She set Caylee on the ground. Then she started digging. She realized that the ground was too hard for her to dig deep enough to conceal the body, so she put the body back in the trunk, and then she used the shovel to break into the shed. She took the gas to either burn the body or to burn evidence (clothing, stuff in the trunk that the body had leaked decomp fluids onto, possibly her clothes). She left the home and disposed of the body. More than likely, she buried it somewhere.
After Cindy retrieved Casey and brought her home on July 15th, after LE left, at some point she told George and Cindy what she did. They engaged in her story about Zanny and hid evidence in the car before LE took it on the evening of the 16th. And they've been lying and obsfucating ever since...to protect Casey. It is very possible that Lee knows, too. If you read the exchanges on myspace between Casey and Lee, it's evident that theirs is an oddly close relationship. Big brother would protect little sister, even though he's disgusted at what she did.
A couple of nights ago, Cindy told Rozzie Franco, "I'm not going to lose another one." They know Caylee is dead, and they don't want to lose Casey--either to prison or to Ol' Sparky (or the needle, if that's what Florida's using these days).
That's my theory, anyway.
