Ok Phansa marks the end of the three-month Buddhist Rains Retreat (Phansa), when monks who have stayed in a single temple since Asalha Puja are free to travel again.
The most photographed version of the day is the Illuminated Boat Procession (Lai Reua Fai) held along the Mekong in Nakhon Phanom, where large bamboo-and-light boats are floated down the river after dark — but temples nationwide hold their own merit-making ceremonies and morning almsgiving.
In Bangkok, expect extra activity and boat processions along the Chao Phraya near major temples. This is a Buddhist religious observance, not a tourist show — dress modestly if visiting a temple.