“I just turned in my book. Huh. Doesn’t quite seem real.”
“I just turned in my book! Time to go out and celebrate!”
“Also, now that I’m not writing I can to do All The Things (that are not writing)!”
“Because even though I just turned in a book I’m Not Tired At All!”
“Why have I been staring at the wall for the past 7 hours?”
“Wonder if there are any second-run movies worth seeing. I could get it together enough to leave the house. Probably.”
“Or I could just take a nap. Naps are good, too.”
Adjust as necessary for the finishing of significant drafts along the way. Also applies to any other significant milestones: finishing a book and sending it to critiquers, finishing a book and starting to query, finishing a final round of editorial revisions and letting the book go for the very last time.