In a 28-day cycle, the first 3 to 5 days are your period. After bleeding ends the first safe period begins, then the ovulation window, then the second safe period. Ovulation usually happens about 14 days before your next period, but can come up to 5 days earlier or 4 days later. Sperm survives up to 3 days, so treat the days just before that window as unsafe too. The second safe period runs from 4 days after ovulation until your next period — the safer of the two.