For updated events listings each month of the year, see Nicky's Gay Pattaya website.
April 18-20, 2013 (and days around these) Wan Lai in Pattaya, elsewhere called Songkran, with slightly earlier date. Held at time of traditional Thai New Year, celebrated by sprinkling water on Buddha, and giving and receiving a dousing from buckets or water guns to/from anyone within range.
Each November, Bali Hai Pier, Pattaya. On the full moon of the 12th month (traditional Thai lunar calendar), people float small candle-lit rafts (Krathong) on rivers, canals, lakes or sea, honoring the Goddess of Water. Fireworks & beauty contests are a big part of the festivities.
December - see website for details on celebrations, gala dinners and other events.