Thursday.

Monday & Tuesday people are either annoyed to be back at work/school and often don't have much energy to want to do something.

Wednesday is good, Friday everyone is either annoyingly overtired that they are grumpy, short tempered, or hyperactive to the point of annoying.

Saturday is probably second best if people have a good sleep. & Sunday everyone is a lazy ****. xD

Thursday, is when everyone sort of relax a bit and have a good time in general. We're all tired but it's in that goofy/funny stage.