Coming from the theory that women who live together will eventually come into a menstrual harmony and go through their cycle at the same time.
Usually they hang out with each other, participating in activities one normally does alone when on their period; such as watching Leonardo DiCaprio movies in the fetal position or complaining about how the cramps this month are worse than any other woman's ever experienced in the history of menstruation.
May: Really? Why, Sue, it looks like we're Period Pals this week! Want to come over to my place and eat some Cherry Garcia ice cream? I think the Sex and the City movie is on on demand tonight.
Sue: I think you know the answer to that.