This is a Buddhist holiday.  Most of the Wats around town should have some sort of ceremony.  The most significant might be Wat Chedi Luang with an evening candle-lit procession around the Chedi.  Besides many of the public, expect to see a large group of monks (last year about 150) taking part in the procession.  Each person will walk around the Chedi three times holding either a candle or incense and flowers.This is a day for Buddhists to make merit, so there will be many offerings, lighting candles, etc.
The Wat next to Chedi Luang (Wat Phan Tao) will have a very photographic ceremony around 7 pm.  There is a moat around this area allowing you to include the reflections of the ceremony in your photos. See photo.

Be prepared for low-light photography.