Characters
Okay, so you're a werewolf. No problem. But hey, three days out of the month, I'm not much fun either.
— Willow · Buffy the Vampire Slayer

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The speaker accepts that someone is a werewolf without fuss, then offers a comparison: for about three days each month, the speaker is not much fun either. The joke treats a monstrous condition as comparable to ordinary monthly moodiness, keeping the reassurance warm and lightly teasing.
Context reviewed