Confused about cycles?

Jordan
So the last two months I started my
​Period on the 6th which was continently on the first Thursday of both months (after missing a period three months ago). I was talking with my roommate and she said it's impossible to start on the same date every time. By Glow, I was supposed to start on the 3rd which would've been a Thursday. But I started today, Sunday which is the 6th.
 How do cycles work?