Pattaya, where it's located

The climate of Pattaya is tropical, hot all year round, with a dry season from December to April, and a rainy season from May to mid-November due to the southwest monsoon. The annual precipitation amounts to about 1,100 millimeters (44 inches), so the rains are not that heavy, if not in the first phase (May) and the last (September-October): this is because the bulk of the rains is released on the Burmese west coast, which therefore protects this area of Thailand. However, some strong downpours can not be excluded also from June to August, although in this period, rainfall remains on average around 95/120 mm (3.7/4.7 in) per month.
In March and April, before the monsoon arrives, there can be a certain increase in rainfall, although the amount of sunshine is still good.
Here is the average precipitation.
Pattaya - Average precipitation

The period before the monsoon, from mid-February to May, is also the hottest of the year: the maximum temperature can reach 34/35 °C (93/95 °F) and the minimum around 28 °C (82 °F). Luckily the breeze tempers the heat. On the contrary, in winter, from December to mid-February, sometimes it can get a bit cool, in fact lows can drop below 20 °C (68 °F) and highs can remain around 24/25 °C (75/77 °F).
Here are the average temperatures.
Pattaya - Average temperatures
Min (°C)232425262727262625242322
Max (°C)313132333232313131313030
Min (°F)737577798181797977757372
Max (°F)888890919090888888888686

The amount of sunshine in Pattaya is good from December to April, the dry period, while it is not very good from May to September, in the monsoon period, when the sky can remain cloudy for many hours.
Pattaya - Sunshine

Pattaya can be reached by tropical cyclones, in particular from typhoons, which come from the east, normally from June to December (although they are more frequent from September to November), and generally affect more directly the interior of Thailand, but sometimes they can also enter the Gulf of Thailand.

Pattaya, coastline

The sea in Pattaya is warm enough to swim in all year round, as you can see in the following table.
Pattaya - Sea temperature
Temp (°C)282829303030292929292928
Temp (°F)828284868686848484848482

Best Time

The best time to go to Pattaya goes from December to April, that is, in the dry season, with a preference for the period December to mid-February because from mid-February to April there is an increase in temperature. From December to February you can bring a sweatshirt for any cool evenings.
If you want to go in summer, in July and August, you will have to cope with sultry heat, increased cloudiness, some showers, sometimes strong, and the risk of a tropical storm or cyclone. However, depending on year and period, the weather can be decent or good for weeks in these months as well.