But the ovulation leads the next period
(rather than following the last), so if a woman has, say, a 35-day cycle
, she would ovulate around day 21.
It's a good idea to keep a record of your cycles, when they start and end (I used to use graph paper) and note any cramping or twinges at mid-cycle. You'll be able to tell your doctor "my cycles are 29 days long" or whatever, and should be able to tell from your changes in cervical mucus, roughly when you ovulate.