Since I was 13 I have always had my period within a day or 2 of the full moon.  When it has been irregular or extended due to clomid, it rights itself the next cycle until it matches up. My closest girlfriends are the same, as was my mom. In some Native cultures this time of the month is often referred to a "being on your moon."  The moon's cycle is 28 days so it makes sense to me.