In many countries you can buy alcohol anytime that you want, but unfortunately Thailand is not one of them. We have specific times selling alcohol, and some days we do not sell alcohol. If you are planning to have alcohol on those days, you should stock it up before.
On a typical day
On a normal day, bars, hotels, and restaurants can sell alcohol when they are open, but in 7-Eleven convenient store, and supermarkets such as Tesco Lotus and Big C, alcohol is sold between 11.00-14.00 and 17.00-22.00 only. So, that means you cannot buy alcohol between 14.00-17.00. But, every rule has an exception. At Tesco Lotus, there is no time restriction for purchase more than 10 litres at at time.
**Update February 2015: According to the current law, now even if you buy more than 10 litres, you have to buy between 11.00-14.00 and 17.00-22.00 only.**
On Buddhist days
Alcohol is not allowed to sell on Buddhist days such as Makha Bucha Day, Visakha Bucha Day, Asahna Bucha Day, and Khao Phansa Day, and usually all bars are close. Instead of trying to buy alcohol, you can choose to enjoy the festivals with us at the temples.
On Election Day
On the Election Day (If I ‘m not wrong, the night before is including), you cannot buy alcohol in Thailand, and usually all bars are close.
Anyway, mom-and-pop grocery stores near your place might sell alcohol to you. But please bear in mind that there are chances that they will be fined (or worst case is get in jail).
During New Year alcohol is allowed to sell. Last year the government was about to ban alcohol selling during New Year Festival because every year there are many car accidents during the festival and almost of the cases are from drunk drivers. Luckily, they know that it is the celebration time and they did not do so.
Hope this information is useful for you. Enjoy traveling Thailand!