The day after is a way of saying something epic or terrible happened the nite before,in reference to the droping of an A-Bomb and it's undeniable impact on ones life. Here are some nites that could lead to a day after

1.Hardcore drinking nite.

2.Ending up in jail for any numerous reasons.

3.Epic sexual conquest.

4.Being on the wrong end of a beatdown.

5.Not remembering last nite at all.