Long cycles- when to take bloodtests? Calling all TTCexperts

Helen
Hi,
​I have long cycles, and ovulate late. This cycle i am doing blood tests on CD 4-5 and 21-23 (progesteron). 
​My cycles is 35-40 days, and i O Around CD  24. 
​Should i wait as long as possible and take the test on CD 23, maybe later- or does it not matter what day i take the test?