Rowling never said. The book says he was trying to remember, then the scene ends without him remembering, and it never brought up again.
Neville was trying to remember what he’d forgotten when Draco Malfoy, who was passing the Gryffindor table, snatched the Remembrall out of his hand.
Harry and Ron jumped to their feet. They were half hoping for a reason to fight Malfoy, but Professor McGonagall, who could spot trouble quicker than any teacher in the school, was there in a flash.
‘What’s going on?’
‘Malfoy’s got my Remembrall, Professor.’
Scowling, Malfoy quickly dropped the Remembrall back on the table.
‘Just looking,’ he said, and he sloped away with Crabbe and Goyle behind him.
At three-thirty that afternoon, Harry, Ron and the other Gryffindors hurried down the front steps into the grounds for their first flying lesson...
Philosopher's Stone - Chapter 9: The Midnight Duel
However, in the movie, he forgot to put on his robe. In multiple shots you can see he is the only student not wearing a robe.
JKR worked very closely with the director, Chris Columbus on the film, so details like this in the flim are likely to be as close to an answer as we will get.
Director Chris Columbus says he's being very careful to work with author J.K. Rowling to get the little details right when translating her book to film.
Archived IGN article from 2001
Karis Ronaldson for Historic Scotland Magazine - Do the Harry Potter films meet your expectations?
JK Rowling: Yes, they do. I mean there are obviously things that are not the same as the books but that is because if you did every scene in the book and translated that into films, the films would be about 24 hours each, so they have got to prune and change things slightly. By and large they meet my expectations.
I have spoken about walking into the Great Hall, I have worked with Chris Columbus, the director of the first two films, and he asked me quite a lot about how things looked, it was really like walking into my own head, it was a very peculiar experience.
Press Conference with JKR on July 16, 2005 - Accio Quote