First Letrozole- cycle help

Rebecca

Hey.. I have pcos with cycles between 50-120 days. My question is... I started my period 5pm today, is tomorrow classed as my first day cD1?. I need to take tablets.CD 2-6 with a scan on day 12

and had afflicting advice on when CD1 is.. today or tomorrow? X so it is the day your start to bleed or the first day of full flow? Thankyou xx