In general, people have their sunday's free. So, if the event is on saturday evening, more people can stay on for longer.
Here's an example:
A player lives on UTC+5 timezone (a bit east of moscow). Event takes place on friday, 18:00 UTC+0. Saturday, the player has to wake up at 06:00 UTC+5, which is 01:00 UTC+0. Let's say, the event lasts 2 hours, which means it ends 20:00 UTC+0. The player can sleep for less than 5 hours.
If the event takes place on saturday, 18:00 UTC+0, he doesn't have to wake up that soon because his sunday is free.