Kathina is a Buddhist festival which comes at the end of Vassa, the three-month rainy season retreat for Theravada Buddhists. The season during which a monastery may hold a Kathina festival is one month long, beginning after the full moon of the eleventh month in the Lunar calendar (usually October).
It is a time of giving, for the laity to express gratitude to monks. Lay Buddhists bring donations to temples, especially new robes for the monks.
As the legend goes, thirty bhikkhus were journeying with the intention of spending Vassa with Gautama Buddha. However, Vassa began before they reached their destination and they had to stop. According to Buddha's guidelines for Vassa, mendicant monks shouldn't travel during the rainy season as they may unintentionally harm crops and/or insects during their journey. As such, the monks had to stop.
Afterwards, the Buddha rewarded the monks by demonstrating a way to practice sharing and generosity. A lay disciple had previously donated cloth to the Buddha, so the Buddha now gave that cloth to the group of monks and told them to make it into a robe and then offer it as a gift to one of them. A frame, called a Kathina, was used to spread the robe while it was being made.