When, soon after a friend's birthday is over, you continue to treat them extra specially, as you did on their birthday, out of a habit you developed during said friend's birthday.
When there is a residue of special treatment left over from one's birthday.
Guy #1: Grab me a beer.
Guy #2: Okay. *gets Guy #1 a beer*
Guy #1: Thanks man, didn't expect you to actually go and get me one.
Guy #2: Birthday residue I guess.