What do you count as 1st day of cycle?

Hi, I'm new here and would like to introduce myself. My DH and I are trying to conceive, me at age 41 and he at 46. I know I should know the answer to this question, but I've gotten so many different answers that I'm confused at this point. What do you guys consider the first day of your menstrual cycle? Is it the first day of bleeding whether it be very light spotting or the first day of regular flow? I'm trying to predict my ovulation day and I'm not sure whether to start counting from the 1st days of light spotting or the 1st day of actual bleeding with regular flow. Any help and opinions would be greatly appreciated. Thanks!