There are certain days of the year in Thailand when sales of alcohol are restricted nationwide. Since 2009, there have been designated ‘National No Alcohol Days’ to include the Buddhist holidays. Under Thai law, alcohol cannot be sold on these days and vendors caught breaking the ban are liable to a prison sentence of up to six months, a 10,000 Baht fine or both. The National No Alcohol Days in Thailand include the most important Buddhist holidays:
Makha Bucha (February/March)
Visakha Bucha (May/June)
Asahna Bucha (July/early August)
Wan Khao Phansa (July/early August)
Awk Phansa (usually in October)
The exact date of these Buddhist holidays varies each year according to the lunar calendar, but you can check here under the Thailand events and festivals section of this site.

Election days
In addition to the dates listed above, alcohol sales are also restricted on election days, including local elections.
Is there a blanket ban on alcohol sales?
In previous years, the alcohol ban on Buddhist holidays has not always been strictly enforced, particularly outside of Bangkok. In rural Thailand, there seems to be a much more lax attitude to applying the law. In popular tourist resorts such as Phuket and Pattaya, some bars have been known to turn down their music, but still serve alcohol on restricted days. Other venues have resorted to selling beer in coffee mugs to bewildered tourists. Local police use their own ‘discretion’ to decide which venues need to be visited and fined.
Stores like Tesco Lotus and 7-Eleven will have signs up on dry days saying no alcohol sales, but not everywhere is treated the same. Hotels are normally permitted to sell alcohol to their guests, but that may change under the current military government.
The Thai government
The military government that was in place following the 2014 coup promoted ‘Thainess’ and traditional Thai values. A campaign called Ngod Lao Khao Phansa (no drinking alcohol during Khao Phansa) was an idea from Prime Minister Prayuth. The former soldier called for Buddhists in Thailand to abstain from drinking alcohol during the 3-month Buddhist Lent period, but his idea never really caught on.