Please see why you should come and visit Costa Rica
Many people recommend that you SHOULD travel in the rainy/green season. For Guanacaste, if you are purposely coming in the rainy season for the lush green growth, be aware that in Guanacaste, there is a reasonable chance in May that there will be no rain - in other words the rainy season may start a month late - so there is a risk that May will be the driest and brownest month of the year, or it may be green, wet and very lush.
Rainy Season - what a horrible sound for a vacation. Especially if you have ever had a week or two of non stop rain ruin your vacation in Hawaii.
Some people have started to call it the 'prime' season and the green season to take away the stigma of the 'rainy' season. In the Guanacaste province, the Dry/Prime/Green season is the middle of November to the end of April. The Rainy/Green season is beginning of May to middle of November.
While I first heard of 'Green season' in 2007 or 2008, it still is not catching in (circa 2018), but people are still trying.
Here is our actual experiences in the Rainy/Green season. All except Day 8 were recorded within 60um of the coast End of July, 2018:
Day 1: Sunny until 5pm, then the rain started (heavy within 30 seconds) and kept rainy until the early morning.
Day 2: Sunny until 4pm, then heavy rain from 4 to 6:30 then a nice evening a couple degrees cooler than the daytime.
Day 3: Rainy in the morning until 7:30am, then sunny until 4pm, then heavy rain from 4 to 6 then a nice evening a couple degrees cooler than the daytime.
Day 4: Sunny until 2pm, heavy rain from 2pm to 4:30pm.
Day 5: Sunny until 4pm, heavy rain from 4pm to 10pm.
Day 6: Sunny until 3pm, heavy rain from 3pm to 5pm, then 7pm to 8pm.
Day 7: Sunny until 6pm, extremely heavy rain until 7:30pm.
Day 8: Sunny all day, but from 2:00pm on we were in Liberia whereas all the rest of the days were within 60km of the coast so I don't know if it was raining on the coast in the afternoon or whether the coast also had a rain free day.
I had a similar experience in 2007/2008 when I came during the rainy season, though I didn't document it day by day.
I could repeat for other trips, like 2019 (which had less rain than 2018), but the point is, the rainy season doesn't mean Hawaii type rain for days and days and days.
Having said all this, 17 days a month in July is the statistically average number of days of rain, so statistically there should have been 2 or 3 days with no rain instead of 1 out of 8. But our experience, with rain every day except the last, was still a pleasant one. We tended to go to bed earlier and get up earlier to take advantage of the dry sunny daytime weather. While it isn't exactly this way, essentially think about the sun as being up from 6am to 6pm all year round.
Reasons to travel in the Rainy season:
Reasons to NOT travel in the Rainy season: