The Custom of Reading the Book of Deuteronomy on the Night of Hoshana Rabbah

Festivals are a time for joy,

See the Aruch Hashulchan (Siman Tashsad, Section 11) who writes: Since an additional mitzvah is added today, which is the conclusion of the festival, and on the festival we are judged concerning water, and water includes sustenance and other matters, therefore, the custom of all Israel is to invoke awakening on this day in two ways: one is like days of judgment, that is, many stay awake most of the night and read the Book of Deuteronomy because of the essential love of Hashem and awe of Him, etc.

 
Blessings,
Hillel Meirs