Costa Rica is a very unique country in that though it is very small, it is comprised of 12 different micro-climates! That being the case, no matter what time of year you would like to be married there is a part of the country that lends itself weather-wise.
Generally speaking, November through May offers the best weather and overall experience in most parts of the country. The rest of the year, chances of rain shouldn’t bother you as there are plenty of sites with roofs, and tents are easily arranged. Mid September through the end of October is considered the Green Season in most parts of the country, with heavier and more frequent rainfall.
There are two parts of the year that should be avoided. These are the two Peak Weeks (Christmas/New Years and Easter) during the year when tourism is at its highest, hotel prices are substantially increased, quality of service is decreased and the most popular destinations are crowded. These factors make for an unpleasant and unsuccessful experience all around.