There are actually no best periods to come and visit San Blas islands. But there are 2 different seasons though : the rainy season (from June to December) and the dry season (from January to May). Some people might think that rainy season is a “bad” season because you’ll get lots of rain, or storms, etc.. but it’s definitely a false statement ! Of course there are chances to get rain, but ….; Because islands are far away from the shore, you’ll get actually only a few chances to get rain, and even though you get some, it won’t last long anyway (usually maximum 1 hour). Actually the “rainy” season could be compared to an alternation of cloudy days and beautiful sunny days, most of the time with a very light breeze, or no wind at all.
On the opposite, the “dry” season (which is supposed to be the high season) is characterized by no rain at all, and sunny days, but…. also by strong and constant winds (called the trade winds, and going from east to west, force 3-4 beaufort), which might be a bit annoying for those of you who like to snorkel (water is a bit less transparent), relax on a beach, etc.. (but of course that will be a paradise for sailing)
To resume : don’t hesitate to come in San Blas during the rainy season ! From our point of view, this is actually the best time to come in san blas… You will most likely have beautiful and sunny days, with no wind, and only short periods of time with clouds and eventually a bit of rain. (Plus, the islands will be less crowdy …)