Do it 5 or 2 hours before the game jam ends or perhaps the day before just to be safe so you can make sure you get your game submitted.

If your game is close to completion. Just submit it the moment you feel you are a 100% done. It doesn't matter too much as long as you do it before the deadline :D

Use the extra time to polish, test the game and fix the bugs. There is always something to do :) and publish some hours before the end!

Agreed with the rest, and also the last time there was a big jam like this, the itch.io servers had some trouble right at the deadline! It's supposed to be fixed, but I have a feeling this jam will test the limits.

For myself, I haven't uploaded a WebGL game before, so I want to get an early build uploaded ASAP to make sure everything works and I know what I'm doing! (I know it doesn't have to be WebGL, but it's a simple game and making it playable in the browser makes more sense and is probably more likely to get reviews.)

I believe the games are shown to you in a random order once the deadline is up so having your game getting lost won't be a problem ;)