Sunday is a classic.
I don't know how it is in Vietnam or other south-east Asia region, but for European countries mostly, Friday evening and the whole Saturday - a time when everyone goes outside to see friends, spend time with family outside, go somewhere outside of the town. And Sunday is a relax-home day when you have free time to relax and make some home routine.
Castle Siege time isn't so late on Sunday, also I've never seen issues with the attendance from your time zone.