The theme will be announced on the page and the youtube channel (as a premiere) by beginning of the jam
Timezone.. GDevelop in France, so it's a perfect time for them (7:00PM), also, I am in Middle East (8:00 PM), HelperWesley (GDevelop's official content creator) in Canada I think (2:00 PM?)
Note that GDevelop Game Jam is an international Competition, It's normal to find the time not suitable for some countries, In any case, you can get up at your usual time, and check it out. Also, everyone will have the same period, the worse the starting time, the better the sending time (you will be able to send the game before going to sleep, unlike some countries where some people may forget to send the game because it is in the middle of the day)
Good Luck 👍