Question about when to start taking letrozole at the right time??

Whitney

I was prescribed letrozole I started spotting 2 days ago still very light my periods are irregular so some months it’s heavier than others as well... I don’t want to miss my 3/7 day Mark taking the pill though! So should I start on the 3rd day after spotting or should I wait until it gets heavier then start from that point?? Anybody else have this similar thing happen?