When to test progesterone to see if ovulating?

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I have polycystic ovaries. Doctor said I could come get bloodwork to confirm if I am ovulating or not. (Not TTC yet, but what to know). They said typically you test on day 21, but my cycles are longer, 33-35days. They said come say 26-28. BUT when I’ve tested with OPKs, I get positives on day 21. Should I go in on day 21 or 26?? I don’t want to go too late and get a negative and freak out.