21 Day Bloods

Kitty
I'm TTC and have just come home from a fertility specilist, I have to get bloods taken on day 21, I usually would have calculated this as 3 weeks to the day that I started my cycle but going by this app it's 3 weeks - 1 day. The first day of my cycle was 20th Feb, so is day 21 Fri 13th March or Thurs 12th? Thanks ladies :-)