Halloween is one of the biggest nightlife weekends of the year—there are dozens of sexy, spooky, and just weird events all over the city. But, make sure you get all your partying in before midnight because at 12:01am the fun police are coming with their fun truncheons (figuratively).
In observance of the Awk Phansa, the end of Buddhist Lent, the ban begins after midnight on Sunday, Oct 29 and lasts until midnight the same day. Alcoholic beverages will not be permitted to be sold at restaurants, bars, supermarkets, or any other venue during this period. The fine is set at 10,000 baht or six months in prison under Section 39 of the Alcohol Control Act.
The ban is no surprise and is one of five booze ban days for religious observance every year, in this case taking place on the 15th day of the 11th day of the waxing moon. 
Recognition of this observance may be in short supply, however, as many Halloween parties on the night are beginning late and have stated that their events will continue long past midnight. 
This dry day is of particular significance as it comes at a time when the Interior Ministry has drawn up regulations to extend nightlife hours in Bangkok from 2am to 4am. Besides the dry days and opening hours, Thailand has a number of alcohol laws that prohibit brewing, sales, and advertisement of alcohol. Last year, hopes were dashed when a long-awaited update to Thailand’s alcohol laws died in the cabinet. The alcohol progressive Move Forward Party May came out on top of the May election—which incurred two dry weekends—but was ousted by the unelected Senate in favor of the current coalition government.