The day after ovulation, the day of ovulation itself and the 3 days prior to ovulation are the fertile days of the woman, i.e., days 11-15 of the cycle approximately.
Sperm can live in the female genital tract for about 3-4 days, so if they arrive before ovulation occurs, they are waiting to meet the egg.
If your periods are regular, count 14 days before you expect your next period and that is approximately the day you will ovulate. Ovulation tests measure the peak of the ovulatory hormone, LH or luteinizing hormone, which rises in levels just before ovulation, so they are a more effective way to calculate your fertile days.
In the case of irregular periods, or months without menstruation, it is advisable to consult a gynecologist if you are looking for pregnancy, because if the egg is not in the fallopian tubes waiting for the sperm, a natural pregnancy will not occur.