The Hebrew calendar is a lunar one and it is based on the monthly cycle of the moon rather than on the annual cycle of the sun. A solar year is 365 days and the months are merely a convenience — dividing the year into 12 roughly equal periods of time. The lunar calendar, however, is based on the renewal of the moon which happens regularly every 29 or 30 days. The lunar year, therefore, is roughly 11 days shorter than the solar year.