Cycle Day 1?

I know this is probably a stupid question but I am so confused on what to count as cycle day 1.

On Monday, I was bleeding a little bit, but not a lot. I only filled one pad all day. In the evening I went without and had an accident so it picked up later on. Blood was red/pink in color, not brown. No blood in toilet, but blood when wiping.

On Tuesday, it was heavy and I had some clotting. Blood in stool.

Which day is cycle day 1? I’m so confused on when to start letrozole because I need to know if I’m on day 3 today or tomorrow. Thoughts?