We're in the Luberon every year for most of the month of May. It's an absolutely beautiful time of year. (See some photos here: slowtraveltours.com/blog/… )
In May the temperatures usually reach the mid 70’s to low 80’s with overnight lows in the mid 50’s to low 60’s. Most days are sunny and bright. You can usually eat lunch outdoors, but probably not dinner. You might experience a brief rain shower or Mistral winds.
What to Wear
You can definitely be yourself in the Luberon... it's in the countryside. People don't dress up much unless they're going to a very special restaurant. Slacks and casual shirts/tops are fine... and jeans too during the day. I wear capris a lot, and my husband often wears hiking pants or khakis.
I'd plan for temperatures that may vary between 55 and 85. This means you'll want to plan on layers that you can take off or add during the day, including some sort of jacket or shawl for evenings. And even though you hopefully wouldn't need it, I'd bring some sort of a rain jacket or small umbrella.
One of the most important things: good walking shoes that are well broken in. Many of the villages are perched, with cobblestone streets and you'll be doing a good amount of walking. You don't need heeled or flimsy shoes. Be sure to have at least one pair of closed toed shoes, as in May you may not be able to wear sandals every day.
We've been spending several months a year in Bonnieux for about ten years and now have a place there. An evening in Bonnieux (or other Provence villages) probably involves going to dinner at one of the local restaurants... which usually start serving at 7:30 or 8 pm and may take two hours or more. If you have an apartment and cook your own meal, you can go for a stroll or maybe have an aperitif or late night drink at one of the village bars. We think it's a good life.
Hope this is helpful in planning what to take on your special trip... I remember the magic of my first visit to this area, and I know you'll have a wonderful time.
Edited: 3:16 pm, January 25, 2013