What happens if I start letrozole on the wrong day?

This cycle is the first time taking letrozole. My Dr is having me take it from CD 3-8. She had me on birth control for the last couple months and I had breakthrough bleeding during the last week of BC, so it's been a little hard to figure out when CD1 is, or if it has actually started yet.

I think yesterday may have been CD1, but I'm not entirely sure. I'm just going to go with it though, so I will be starting letrozole tomorrow. Then she wants me to use OPKs CD 10-16.

So will this cycle be a waste if I got CD 1 wrong?