Periods will be absent for that 21 days. The gap of 7 days is given for the withdrawal bleeding only. You should start taking the pills from the first day of your menstrual period and restart the pills from first day of next cycle even if it's less than 7 days gap. In the very rare and unlikely event when you get the periods during 21 days, may be in initial months, start a fresh course of 21 days as period starts.