As a tourist on a 14-day holiday, you probably won't notice the Thai holidays. Major malls, shops and most of the entertainment for tourists are open all day, all year round.
However, nightlife may be affected by holidays because on some days they forbid the sale of alcohol; meaning that the bars and nightly entertainment close those days. Read more about the holidays with no sale of alcohol in the Buying Alcohol in Thailand article.
If you need a visa extension or for other reasons need to contact the authorities in Thailand, then be aware that they close on holidays or the day after a holiday – if the holiday is on a Saturday or Sunday, then they close Monday instead.
You should pay particular attention to closing days in connection with the Thai New Year, Songkran, where the offices may close for most of the week.
Below, you will find links to lists of the holidays in which public offices close: