Weekend Starts When?

i'm always curious about when does the weekend start? friday night or saturday morning? remembered that i had my departmental dinner two friday nights ago... called the restaurant to make a reservation... the guy told me that they don't accept reservations on weekends... and i conveniently assumed that the weekend starts on friday evening, after work... hehe...

if go by that theory, sunday evening is not part of the weekend? or is it?

complicated sia...