Ah, now I was under the impression that the day was sort of important - they want to get you in the middle of your luteal phase as that is when progesterone should be peaking and indicating ovulation (or indeed, lack of). I thought that if they test too early or too late it might be rising/falling.
I did find this on a New Zealand fertility associates website:
Why Day 21?
This day is chosen on the assumption that the women are having a 26-30 day cycle, and luteal phase is normal and lasts about 10-14 days after ovulation. Thus, assuming ovulation is between Days 12-16 , Day 21 is the middle of the luteal phase when the production of progesterone from the corpus luteum gland peaks in an ovulatory cycle (and if there was then pregnancy, the HCG would prevent the corpus luteum from then dying and it would produce even more progesterone). If ovulation has occurred later than Days 12 ? 16, then the progesterone check would be better done a few days later so as to strike the middle of the luteal phase. Conversely, if ovulation has occurred earlier than Day 12, an earlier progesterone test at about Day 18 is required.
Progesterone levels should be interpreted with respect to the number of days before the onset of the next period. Luteal function ( and hence ovulation) cannot be assumed if the period starts < 5 days later or > 9 days after the blood test.
Possible interpretation of results assuming the test is properly timed are:
1. Progesterone > 30nmol/l ? ovulation has occurred and the corpus luteum is producing sufficient progesterone to induce adequate secretory changes in the endometrium to assist implantation.
2. Progesterone 3-30 nmol/l ? ovulation has occurred > 10 days ago or < 5 days ago, or if ovulation did occurr 5-10 days ago, the level is probably not adequate to sustain endometrial growth for satisfactory implantation.
3. Progesterone >120 nmol/l ? two or more follicles have been recruited.
4. Progesterone < 3nmol/l ? no ovulation in previous 14 days.