Blood test help

Becky • My angel 18-11-17 👼🏻 My little rainbow 17-09-18 👶🏽🌈
The doctors give me the go ahead to start doing tests to look into fertility issues, and they said I have to have 2 blood tests done: day 3-5 and day 21. 
How the hell do I work it out? I have a 33 day cycle on average. I just nodded along because I was so excited I'm going to start getting answers and didn't absorb what she said 😂😥 x