The flooding of the Nile River was the most important annual event in ancient Egypt. As the river burst its banks the flooding, or 'inundation', irrigated farmland and renewed the fertility of the soil. Together with the lunar cycle, this mostly-predictable inundation provided the basis for the Nile calendar. The calendar was divided into three seasons, each with four months of thirty days each. Slowly, the calendar fell out of step with the seasons until the date of the inundation began to fall within a calendar period of drought. This inaccuracy was eventually corrected with the introduction of the leap year during the later Roman period.
