What Are The Side Effects Of Using Novelon Tablet?

Hi ..I have been prescribed to use novelon as a contraceptive.. As per the prescription am supposed to start the course from second day of my periods and continue for 21 days. My periods are very irregular(say once in 2-3 months) Should i still continue the usage on second day of my periods or should i use it every month for a period of 21 days to have sex without conceiving