Confused about start date for birth control (i asked this before but there was a pretty critical typo lol)?

was prescribed Tri-Cyclen Lo, and was told to do the Sunday after period start. I'm supposed to start my period on December 24th, so I would end up taking the first Pill on December 28. However, if I do that, then I'll be on the inactive pills by January 18th, and my period SHOULD be January 24th(I'm very regular). Is the Pill SUPPOSED to change the day you start your period? I'm really confused and can't get ahold of anyone at the pharmacy.