“The appointed seasons of the L-RD, which ye shall proclaim to be holy convocations, even these are My appointed seasons.” – Leviticus 23:1-2, JPS 1917 Tanach
On “the fourteenth day of the first month,” the Pesach offering was made (Leviticus 23:5). A seven-day observance begins on the fifteenth of Nissan, when we refrain from eating chometz, during “the Feast of Unleavened Bread” (Leviticus 23:6). “Ye shall bring the sheaf of the first-fruits of your harvest” (Leviticus 23:10, JPS 1917 Tanach). This was brought to the kohein [priest], on the day after the first rest day of Pesach. The offering is referred to in Torah as the waving of the Omer; it was only enacted after B’nei Yisrael entered the Promised Land.
“Even unto the morrow after the seventh week shall ye number fifty days; and ye shall present a new meal-offering unto the L-RD” (Leviticus 23:16, JPS). That is, fifty days were counted from the second day of Passover, onward until on the fiftieth day, the first wheat offering of the harvest was brought “unto the L-RD.” (The offering that was made prior to this – the Omer – on the second day of Passover, was the first of the barley harvest). Today, we refer to the fiftieth day after Passover as Shavuot, in commemoration of Matan Torah (the giving of the Torah).
In Autumn, we celebrate Rosh HaShannah, the Jewish New Year, and Yom Kippur, the Day of Atonement. Sukkot, which follows Yom Kippur, is considered a Festival, like Passover, and Shavuot; so, it is the third of the Festivals: “Ye shall dwell in booths seven days” (Leviticus 23:42, JPS). We build sukkot (booths) to commemorate the protection we received from the Clouds of Glory, while dwelling in booths, during our forty-day sojourn in the desert. On the eighth day, we celebrate Shemini Atzeret, symbolizing Olam Haba (the World-to-Come).