Gareth, chat to Rob and Mark; both great guys that truly know their stuff. Let them know what your targets are and they can advise best. The season runs from October to end of March/early April with a break in between for the rainy season.
Generally speaking, what you want is rain, lots of it, at the start of your week(s) there, that switches the fishing on big time. Oct - Mar is actually "dry season", but being very close to the equator in the second largest rain forest in the world, rain falls throughout the year, just less predictably during this period.
My last visit was in November 2016, the first couple of days there it hadn't rained for a week or so and the fishing was not that amazing. Then one morning we stayed put and watched a massive thunderstorm unfold...after half a day of rain all hell broke loose and I had the best surf fishing I've ever experienced; by miles.
I want to target tarpon, if I only caught one fish all week and it was a tarpon, that would be a success in my book. So I chose my dates accordingly - it'll be new moon when I arrive, which means good tidal movement, but not as extreme as on the full moon and, being relatively late in the season, the odds are in my favour for some heavy rain. Tarpon in Sette want a bit of current and dark stained river water washing into the surf, so hopefully that's what I'm going to get.