Many women with PCOS have irregular periods (usually long periods in between each cycle) and this is because you are not ovulating regularly. The first day is usually when you have proper menstrual flow. PCOS is the only condition when women do not have periods but have adequate oestrogen production.
If you don't have a period and shed the lining of the womb regularly, the lining can build up leading to a heavy period. Also, if the lining of the womb doesn't shed and becomes very thick, in may confer an increase risk of womb cancer in the long term.
Hence, it is recommended that women with PCOS have at least 6 periods a year. If you have less than this, your doctor will give you some progesterone tablets (eg norethisterone) for about 7-10 days, and you will get a period a few days after.
I hope this helps!