There are only two seasons in Costa Rica: the wet season (from May to November) and the dry season (October-June). 

No matter the season in Jaco, it will always be HOT.  Add humidity into the mix and you get a very "tropical" feel in this low-lying beach town so consider a hotel with AC and/or pool so that you can relax away from the beach or main road as heat becomes intense by midday.

When deciding which season that you would like to spend in Jaco, you should consider the following: 

 The dry season is also the high season for tourists so, more crowds on the beach, waves, bars etc and higher prices for hotel rooms. 

 The wet season is a bit of a misnomer as it there are usually showers in the afternoon; however, both the morning and evening tend to be clear.  This is also the low season for tourism yet it is a great time for surfers as waves pick up intensity in these months.  (be careful swimming in both Jaco and Hermosa during this time - see "Water Safety in CR")

 To enjoy Jaco in any season, stay hydrated and lathered in sun screen and aware of the powerful waves/currents at the beach.