Cultures around the world set aside special days every year as festivals, or times of remembrance. Modern examples (Worker’s Day, Mother’s Day, Independence Day, Valentine’s Day and others) all last 24 hours and take place on the same date every year. But the really old festivals, as observed by world religions, are often a little more flexible. Their dates can vary from year to year, sometimes differing by as much as a month. What gives?
Easter, and related festivals
In the Christian Calendar, festivals are either timed according to a fixed calendar date (Christmas, for example), or in relation to Easter. Ascension Day is celebrated forty days after Easter, while Lent is a forty day period of fasting ending on Easter. Here’s the first complication: Since Sunday’s are holy days, people celebrating Lent do not need to fast on those days. Accordingly, the calendar duration of Lent is actually forty days not counting Sundays, which is usually forty-six days. And then there is Good Friday, which is the Friday directly before Easter. So when is Easter celebrated? The official formula, as determined by the first council of Nicea in 325 AD, is that Easter shall fall on the first Sunday after the first full moon after the March equinox. In other words, Easter can fall any time between March 21 and April 25. But before you go making your own calculations, you need to note that the formula relies on slightly different definitions of astronomical terms. Full Moon, in this case, means “fourteen days after the New Moon”, and the March Equinox is assumed to always fall on 21 March. Both of these are merely approximations of the true definitions, to prevent errors and disputes.
The Jewish festival of Passover (or Pesach in Hebrew) is an interesting case: It’s date is determined according to the Lunar calendar, which does not correspond to the standard Civil Calendar. In the Jewish Lunar calendar, a month starts immediately after New Moon, when the first sliver of the Moon’s crescent is visible. Since the phases of the moon follow a cycle of about 28.5 days, each Lunar month lasts either twenty-eight or twenty-nine days. The problem with this system is that the Lunar Cycle runs 12.4 times per year, so that the Lunar Calendar slips out of sync with the seasons quite rapidly. For this reason, a thirteenth Leap Month is inserted every few years.
Islam also follows a Lunar calendar which, unlike the Jewish calendar, is not synchronised with the seasons. Accordingly, the Islamic Lunar Calendar needs no correction, and has no leap months. The Islamic year thus is shorter than the civil year, measuring twelve Lunar cycles and not one Earthly orbit. As a result, a specific Islamic date will wander all over the civil calendar as the years pass. Ramadan is actually the ninth month of the Islamic Lunar Calendar, and the entire month is considered a holy festival in which all able-bodies Muslims must fast from sunrise to sunset.