“After eating unleavened bread six days, you shall hold a solemn gathering for the
Lord your God on the seventh day; you shall do no work.” (Deuteronomy 16:8)
The last day of Pesach falls on Shabbat so the regular parasha, or portion, is pre-empted by a special holiday reading. The selection describes a variety of communal practices and closes with an admonition to observe Passover: “Observe the aviv month and offer a passover sacrifice to the Lord your God, for it was in the month of aviv, at night, that the Lord your God freed you from Egypt.” Aviv means spring and this is why Passover is also called Chag Ha-aviv, the Spring Festival. It also is the reason for the Jewish calendar’s complexity.
Jewish months contain either 29 or 30 days because they follow the lunar cycle, which is 29 ½ days. However, twelve lunar months (354 days) are about eleven days shorter than a solar year (365 days), while thirteen lunar months are about 19 days longer. Over time, the accumulation of either disparity would cause the holidays to “float” throughout the seasons. This isn’t allowed, though, because the Torah commands Pesach to always be in the spring month.
The solution is a system of leap years in which an extra month is added to “give back” 29 days and keep Pesach in the spring. In a Jewish leap year, an additional month of Adar (the month before Pesach) is inserted into the calendar (the first Adar is the leap month). Such a year is called a Shana M’uberet, a pregnant year. While the Gregorian calendar’s leap year is easy to keep track of (every four years), Rabbi Hillel’s fourth century standardization of the Jewish calendar is a little trickier: seven leap years in a nineteen-year cycle.
Gut Shabbos-Gut Yontif/Shabbat Shalom-Chag Sameach