Here are the rules for days and dates. Something so simple shouldn't be so complicated, but every damn time someone says at the stroke of midnight "Hey! It's (insert next day of the week here)!" I just want to hit them. Hard. So, without further adieu, here are the rules:
At the stroke of midnight, the next day has not begun. If you went out on a Friday night and you're still out after midnight, it's still Friday night. Even if you stay in, it's still Friday night until you crash out. If you are part of a collective, it is not the next day until the overwhelming majority of the group has crashed (i.e., lans).
The other rule is that the next day officially begins when the sun rises.
Questions about the rules are welcome, but if you have issues with them you can suck it, but after midnight tonight.