The oral contraceptive pill that you have been prescribed (Mercilon) is a 21 day pill with a 7 day pill-free break.
This means that you have to consume 1 oral contraceptive pill daily for 21 days and then have a 1 week break (7 days) before starting a new pack.
During this 7 pill-free days, your period should come. If it comes exactly 7 days after your last active pill, this means that your cycle is a 28 day cycle like the majority of women who are on Mercilon.
if it comes 5 days after your last active pill, you then have a 26 day menstrual cycle.
Regardless of when your menses comes sometime during these 7 pill-free days, you have to start the new packet of Mercilon exactly 7 days after your last active pill in order to prevent pregnancy and to have a regular menstrual cycle.
If your menses fails to come at the end of the 7 pill-free days or if you have missed taking your oral contraceptive pill for a day, you have to see your gynaecologist for advice as to what to do next.