The Azorean weather is known for being temperamental. It's a really common saying among locals to say that you can 'experience all the seasons of the year in just one day'.
The best time of the year to visit would be the summer, where the temperatures vary from 22ºC to 30ºC and it's less likely to rain. But don't expect beach weather every day! It's still possible to rain during the summer and if you stay for a week it's possible you'll have at least one rainy day. But don't worry, you have plenty of things to enjoy, despite that.