we go for 3 weeks may into June, last year was especially bad at the start of June, it rained solid, for nearly 3 days, not just an odd shower, the temps also dropped considerably.It was also raining in the evenings as you went further into June(not sure if this was a one off)
The year before we noticed it starting to rain more as June hit. I think personally May is the better time to go, and I will certainly be scheduling my family trips now for 3 weeks in May as opposed to 2 in May one in June.
Jules