Hi Swarbrules,
Peer Review and Weekend Peer Review generally close for entires at 12 noon uk time on the Friday and Sunday to allow voting to start and then usually wrap up at 6pm. The setter has some discretion so may chose 7pm if they know they will be late.
Clueless starts Monday evening at the setter’s discretion and generally finishes at 6 or 7 on Wednesday.
Starting early evening accommodates time zones in the US, Europe and Australia so gives everyone an opportunity to get in early if they so wish.
The next challenge normally starts after the previous one has finished to prevent overlap, but this is not always possible of course.