Regardless of anything else, I think it should be a Saturday because most people will not be able to stay up early to finish on a Monday; having to go to work/school later that same morning. I was grateful for the day to relax after staying up so long for a single task.
It’s a 24-hour contest worldwide contest that will probably start at noon, Greenwich Mean Time. When it ends depends on your time zone. Keep in mind though, that you don’t need to use all 24-hours, you can finish on Saturday if you want.