When a group of females who live together or near each other, or at least see each other on a regular basis, start getting their periods at the same time each month. (It's got legit scientific basis. Don't question it) This is usually coupled with nonstop public discussion of cramps, bloating, and how fucking stupid all of the men you know really are.
Guy 1: What the hell is up with those chicks lately? They've been huge bitches.
Guy 2: Oh, dude, they're all on the rag. It's that communal menstruation bullshit. I'd stay away if I were you