Congratulations! We won't let the 'cat out of the bag'! (being an elopement, I assuming no one knows!).
Generally, this is the beginning of the rainy season, but having said that, you will likely get rain in the afternoon and/or overnight. It will still be warm, but humid. Plan for sun in the morning and early afternoon so you will want to do any hiking or outdoor activities during this time. You won't need any heavy, warm clothes. A fleece could come in handy if you get a chill, but you will only need that when you are in the central valley (San Jose) because you will be in the mountains where it is a bit colder.
As for how much rain you will get....??? Good question! It could be a down pour for 10 min. to a couple of hours to overnight....! Your guess!