i usually do NOT train on my first day...
2 reasons for this...
1) my period is so heavy that i have little strength in the gym so i can't push my normal weights and i tire very easily...
2) in my personal experience, when i train on the first day, it seems as though my period for that month gets a bit screwed up... Like i will bleed for the first 2 days, then stop for 2 days and then bleed again for a day or two and then in about another day i spot... so it doesn't seem like it flows normally... do NOT ask me why, i have no idea lol... just what i have experienced...
But #1 reason is valid for most women that have a heavy first day...
super