Kathina is a holiday that occurs typically in October or November, at the end of the Vassa time, when Buddhist monks and nuns have been in seclusion for three months during the rainy monsoon season.
Kathina ceremony is when they come out of seclusion. It is celebrated by the offering of new robes and the giving of other necessities to both the monks and the needy. This celebration, Kathin, can last for an entire month by the Theravada Buddhist as a time of expressing gratitude to the works of the Buddhist monks.
It is observed differently in different countries and cultures with each having its own traditions and celebrations.