The Gregorian solar calendar used by the Western world, is based on the cycle of the sun. The tropical (solar) year is 365 days, 5 hours, 49 minutes and 16 seconds. Thus every four years an extra day is added to the year at the end of February in order to compensate for the loss of 5 3/4 hours each year.
Unlike Western society, Jews, Muslims and the Chinese all follow a lunar calendar. Like the solar calendar, the lunar calendar has 12 months, with each month measured by the waxing and waning of the moon.
Because the lunar calendar is only 354 days long and does not correspond to the solar cycle, the lunar calendar will not relate to the seasons unless the extra days on the solar calendar are accounted for. If not, a lunar month might occur in spring one year and in winter the next. The lack of coordination between the lunar months and the seasons would not be such a problem for the Jewish calendar, except that it results in a direct contradiction to a Biblical command: “Observe the month of Aviv (Spring), and keep the Passover for the Lord your God; for in the month of Aviv, the Lord your God brought you out of Egypt by night” (Deuteronomy 16:1).
Consequently, the holiday of Passover must be observed in the Spring. To accomplish this, the month of Adar is doubled during a leap year (Adar I and Adar II). Why Adar? Before the Jewish calendar was fixed by mathematical calculation in 350 C.E. (approximately), the new month was determined by the Sanhedrin based on the testimony of witnesses who had seen the new moon. Likewise, the Sanhedrin declared the leap year based on their observations of the season. Adar, the last month before Nisan (the month of Passover) was the deadline for the declaration of a leap year.
According to the current calendar, a Jewish leap year occurs seven times within every 19 years. This year, 5771, is a leap year (today’s date is 3 Adar I).
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