Boon Ok Phansa (บุญออกพรรษา) is Month #11 of Heet Sip-Song – The Twelve Isan Merit-Making Traditions.
Ok Phansa ("Leaving the Rains Retreat") is the final day of the three-month Buddhist Rains Retreat, which is fully explained with Boon Khao Phansa, the retreat's first day. Monks have special meetings with their abbot to discuss the rules of monkhood, while for lay people it's a day for making donations, listening to sermons, and racing boats.
In Isan this heet used to be called Boon Jut Prateep ("Candle Lighting Ritual") because in the evening people make light displays with small candles or oil lamps. Usually they were set on some sort of frame made of natural materials like bamboo, banana plants, and palm fronds (and sometimes still are) but they might also be placed on steps, laid out around a shrine, hung in trees, or just set on an ordinary table. Along the Mekong and other rivers, most famously in Nakhon Phanom province, the lamps are put on giant bamboo rafts, up to ten meters long, with thousands of lights making large, elaborate pictures. Nowadays, sometimes electric lights are used for these rafts.
Ok Phansa is also the day of bang fai pa-yah-nahk ("naga fireballs") which are seen in many places along Isan's stretch of the Mekong, but mostly it happens in Nong Khai province. Locals believe these are an offering from the resident naga to the Buddha and they outright reject any suggestion of trickery. The fireballs do, however, look a lot like tracer bullets; but that's a story for another post.
Boon Ok Phansa happens on the full moon day.