What Is the Buddhist Rainy Season? Understanding the Vassa and Kaṭhina Ceremonies Each year, Theravāda Buddhist monks all over the world make a determination to stay in a single monastery for a three month period of the Rainy Season also known as Vassa, or the Rains Retreat. You will hear the term “Vassa” often in this article, so please remember it. Traditionally, it starts the day after the Full Moon Day in July or the day after the Āsāḷha full moon known in the Pāḷi language.