Normally, your period will begin the last week of your pill pack. The pills are usually placebo pills that are placed there, and this is when it begins. Depending upon when you start your pill pack, it is hard to definitely tell what certain day your period will begin in April.
Birth control pills work on a four-week schedule, much like a normal cycle in women. You take three weeks of pills and then you're off one week for your period. Not knowing where your cycle ends this month, it's impossible to give you an answer for April. Try the online period calculator at mymonthlycycles. com.
Your period will start roughly 28 days from the end of your last one. Starting up birth control will not alter when your cycle starts next, it will just help to regulate it.