Period day changed while on birth control

Patient: I am 19 and I have been on Aviane for 3 years now, taking it every night at 10pm. My last pill is on a Monday, and I used to get my period on Thursday. About 6 months ago my period started coming on Fridays. And last month it came on a Sunday, and I am now waiting for it to come this month. Why does this happen and should I be worried about its effectiveness?