On the first day you shall take the product of hadar trees, branches of palm trees, boughs of leafy trees, and willows of the brook, and you shall rejoice before Hashem your God seven days Leviticus 23:40 (The Israel Bible™)
Sukkot, the Feast of Tabernacles, was the most joyous of the festivals observed in the Beit Hamikdash (Holy Temple). According to the Sages, on Sukkot, however, Hashem (God) judges the people for water and determines how much rain will fall in the coming year. Given the fact that the Land of Israel is very dependent on rainfall, it seems that Sukkot should be a solemn time and not one of joyous celebration. What is the reason for such festivity? By making His people dependent on rainfall which comes from heaven, Hashemensures that they must maintain a close connection with Him through prayer at all times. It is the constant connection with the Almighty, by virtue of His children’s continued dependence on Him, which is the cause for great celebration on Sukkot.