What Causes Periods A Day Later After Finishing Birth Control Pills?

Hi! I started my birth control on a friday. However, whenever I finish the pink pills which my last one would be on a thursday to start my period on a friday (since that's the day I start a new pack each month), instead my period starts on the saturday. That gives me one day of taking the fake pill before my period starts. I always take my pills on time and never get pregnant but I was just wondering if it's normal to start your period a day later or what's the cause?