It'll probably be the first day. I'll be there lying on the beach, semi naked, desperately looking round for something to mark my progress while I go to the bar. I suppose I might just snap the cover back to break the spine, but the trouble with that is the book always opens at that page. When you've moved on it keeps dragging you back to the same place. Ripping bits of the cover off is just convenient.
A new method I've discovered since my original post is to rip out pages I've read - say about twenty pages before. It has the advantage that you can leave the book lying around when you've finished it. Then when people pick it up to read they get to that page and... On the plus side. this encourages them to purchase another copy so it's all good for the author - Win win. Obviously when I've finished the book the last page is the first to go - That sounds odd, but you get my drift.