ovulation dates and day 3 and day 21 blood test

Has anyone got a day 3 and day 21 blood test? What exactly are they for? been trying a few months but think we had the wrong dates. only used a ovulation kit last month for the first time. The dates we had before xmas were all wrong so I'm praying it was just us getting dates wrong. So I did ovulate last month. is this a good sign at least? It's getting me down. do i need the day 3 and day 21 test. I'm new btw 😀