Be aware of public holidays if you need to interact with officials in Thailand, like if you want to extend your stay at immigration.
As an ordinary tourist on a 14-day vacation, you probably won't be affected much by the Thai holidays. Shopping centres, stores, and most common attractions are open every day, all year round.
Although, the nightlife can be affected by holidays because, on some holidays, shops and bars are not allowed to sell alcohol, which is why bars and similar establishments are typically closed on those days.
However, if you need to extend your visa or, for other reasons, need to interact with public authorities in Thailand, you should be aware that they are closed on a wide range of holidays. And unlike in many other countries, the employees do not risk missing a day off if the holiday falls on a weekend or in case of two colliding holidays. In that case, they get a compensatory day off after the weekend/holiday.
You should especially pay attention to closure days during the Thai New Year, Songkran, where offices can be closed for almost a week.
I have made annual overviews of the holidays and observances when public offices are closed: