What is the difference betwee28n 21 day birth control pills?

Not much. Both types have 21 active pills taken during the first three weeks of your cycle. The extra 7 pills in the 28 pill pack are inactive. When the 21 pill pack runs out, you wait 7 days then start a new one. With the 28 pill pack you start a new pack as soon as the old one runs out. It both cases menses occur during the 4th week due to hormone withdrawal.