If you miss one active pill( hormonally active pills are one of the 1st 21), take the missed pill as soon as you remember and then take your next pill at the regularly scheduled time.
If you miss 2 active pills in a row, what you do depends on where in the pack the missed pills are. If the missed pills come from the first 2 weeks, take 2 pills the day you remember and then take 2 pills the next day as well, then finish the pack as you normally would. You must use a backup birth control method for 7 days after the missed pills.
If the missed pills come from the third week, throw-out the rest of the pill pack and start a new pill pack that day. You must use backup birth control for 7 days after the missed pills. Also, note that you may not have a menstrual cycle that month. Of course, if you use Sunday as a reference, continue taking one pill a day until Sunday then throw out the pack and start a new pack on Sunday.