So we start with menstruation. The date used for this by everybody is the first day of bleeding of the period. That’s day one. This lasts for 3-5 days. Next comes the follicular phase, where the lining gets rebuilt for implantation.
After ovulation is the Luteal phase which lasts between 10 and 16 days until the next period starts. Measuring this time period with tests can give you a reasonable predictor of future ovulation. This period+period length-luteal length=ovulation day.
Fertility is highest between 5 days before ovulation (lifespan of sperm), and two days after ovulation. Another measure for this is next period minus 10-19 days. A simple calendar is available online.
Once you hit the jackpot, you have a shiny happy baby in 266 days.