Are you under any particular stress? Stress can make your period come and go at different times. It can also make it lighter or heavier. Also, have you been hanging out with any particular female a lot lately like a new friend or old friend, sister, etc? Girls' periods can "synch up" when hanging out often. So maybe yours and hers are trying to catch up to each other so they're at the same time. But either way, as long as you're getting one every month and it seems normal other than the day change, there's probably nothing to worry about. Sometimes they just slightly change and come a different day. And again, there are things like stress, etc that can affect it as well.