What day do you count as first day of cycle?

I am 33 and have been TTC for 9 months. Does anyone know what day you should count as the first day of your period? Is it when it is light or the first full day of heavier bleeding? I am curious because I may be mistiming my ovulation timeframe. I usually am very light for the first day (sometimes 2) before full bleeding starts but I always count the day I have any blood as Day One. Is the incorrect? Should I wait until the day it gets heavy to count?