I've never really referred to it like this, because I try to stay prim and proper. er.
Because if a typo a while back (I can't remember whose exactly, maybe one of my friends can help out there) Thurday has become Thrustday.
We use Thrustday to misbehave. To act crazy. To flirt. To use innuendo for fun.
It's goofy, but it's fun, and it breaks up the monotony of the week. Everyone should celebrate Thrustday every week.