A couple of times I can remember, Cath and I both got our periods on Cinco de Mayo, and someone (maybe it was me?) called it Syncho de Kayo. Well, it hasn't happened - again - lol. I really thought 2016 would be the year, because mine came May 2 and Cath has been a few days after me lately. For a long time - maybe 5 or 6 years - we were within a week of each other. Sometimes me first, then over a few months, Cath first. We started on the same day more times than I can remember. But as we've gotten older, Cath's period is erratic, and mine is still sort of "average" timing. I'll get 12, she gets 11. So we're all over the place... but this year, I thought for sure, we're gonna both be on for Syncho.
But... nope. Darn! Put the pinatas away, maybe next time.
Menstrual synchrony is an interesting concept. Apparently, there's no scientific evidence it occurs, but there seems to be plenty of anecdotal evidence. According to Wikipedia "most women report that they are aware of menstrual synchrony and nearly as many report personally experiencing the phenomenon".
Cath and I were within days for many years... more than coincidence, I would guess. But, I also think it's one of those 'selective memory' things...you forget all the things that don't agree with your memory, lol. After the kids, my periods were very erratic for a while, and seem to have settled down to about 27 days. Cath seems to be around 29 days, but also skips sometimes. (She is NOT pregnant, lol). I would love to skip a few now and then. She sees our doctor regularly, who doesn't seem worried. I think it's stress - work, money, the usual. More wine, I say.