Panama [1] has two seasons, rainy and dry. The dry season lasts mid-December–mid-April and is Panama’s “summer.” This is the high season for tourism. The weather can be especially lovely at the beginning of the dry season.
The rainy season lasts mid-April–mid-December. During most of the rainy season, storms pass through quickly, leaving much of the day and evening clear. Storms tend to start earlier and last longer in the final months of the rainy season; this is not a great time to go on long hikes or drive on rough roads, as the countryside turns to mud and routes can be impassable.
The rain never stops completely in some parts of the country. Be prepared for some precipitation year-round along the Caribbean coast, in the western highlands [2], and on the islands of Bocas del Toro [3]. September–October and February–March are the driest times in Bocas. Late February–March is considered the best time to visit the islands of Kuna Yala [4].
Temperatures in Panama don’t change much during the year. In the lowlands, expect a high of about 32°C (90°F) in the day, down to 21°C (70°F) in the evening. It never gets cold in the lowlands, and the dry-season breezes in the evening are pleasant. It gets considerably cooler in the highlands. Panama can be quite humid year-round, but especially so in the rainy season.
Panama’s biggest holiday is Carnaval, held each year in the four days leading up to Ash Wednesday. The country comes to a complete halt during those days. November is filled with fiestas patrias (national-independence holidays).
Links:
[1] http://www.moon.com/destinations/panama
[2] http://www.moon.com/destinations/panama/western-highlands
[3] http://www.moon.com/destinations/panama/bocas-del-toro
[4] http://www.moon.com/destinations/panama/kuna-yala