Hi purple,
The first day of menstruation is referred as day1 of the cycle. The length of the menstrual cycle is usually around 28 days, but it varies from 21 or less to over 35 days in a long cycle. Basically, whether your menstrual cycle is long or short, you would probably ovulate 14 days before the beginning of your next period. In other words for a short cycle of 21 days, ovulation will occur around day 7 and for a long cycle of 35 days, ovulation will take place around day 21.
You can also based on your cervical fluid. Approximately 5 to 6 days before ovulation the mucus starts to flow into the vagina. Your fertile time begins when you feel a sensation of discharge from the vagina. The mucus usually starts with a day or two of sticky mucus, which becomes increasingly creamy,wet,slippery and sometimes like egg white. As ovulation approaches, the mucus become more fluid and thinner. This mucus contain an elastic gel-like substance. Sometimes you can even stretch between fingers for a few inches. This fertile mucus helps to protect sperm from acid secretions in the vagina, nourish and keep sperm alive for several days and aids sperm migration.
Therefore predicting when ovulation occurs and based on the cervical fluid you can plan your BD session accordingly and increase your chances of getting pregnant.