Ovulation happens when one of your ovaries releases a mature egg (ovum) that will await fertilisation. The egg is then available for fertilisation. The fertilisable lifespan of an ovum is approximately 24 hours. The fertilisable lifespan of a sperm is 48 hours. The most fertile days in the menstrual cycle are 12 to 16 days before the start of the next period. Try out this handy ovulation calculator.