Why is this test done on Day 21?
The timing of ovulation is associated with a peak in progesterone, and that spike is related to the subsequent menstrual period - not the preceding one.
In an average cycle (28 days), the time between ovulation and the next period is about two weeks, so progesterone is measured about seven days before the expected period, or on Day 21.
If your cycle is longer or shorter than 28 days, the test day will be adjusted accordingly. For example, someone with a 35-day cycle would be tested for progesterone seven days prior to the subsequent menstrual cycle, on Day 28.
When calculating your Day 21, make sure you're starting from a true Day 1, which is the first day of full flow (don't include spotting).