I have to reschedule my trip in December to sometime in March, April or May. I will be staying in Arenal and Alajeula for 6 days. Which month would grant me the best weather? Thanks for all of your help!!

You have some replies on your other post. Have fun!

I am not seeing any replies. Can you just tell me if April will still be green enough and not too hot in the areas that I am intending to visit??? Thanks for all of your help

Arenal and Alajuela will be green in these months. It is the drier areas of Guanacaste that tend to be brown and dusty after a few months of little or no rain. The central areas are always nice and green and the temp just climbs a little bit during these summer months of dry season. Avoid Holy Week, which I think is early this year--last part of March, maybe? That particular time is very busy and not the best time to visit. Other than that, March, April, or May should be very similar with the rains starting up gradually in May. Personally, I like to be there when there is a little afternoon rain!

If you're curious, you can surf over to weather.com and type in the city names for Liberia, CR and San Jose, CR and look at the annual average temps for each to compare. Liberia being in the Guanacaste region that Shillmac mentions will be warmer and drier, with fewer inches of rainfall per year.
Having said all that, April in Arenal and Alajuela will be fine for your stated concerns.

Cheers!