The weather is always bad on midsummer's day. I think it's a requirement. Obsessive reading of the weekend's weather forecast is a national sport.
However, I think Margaret and Janne might have caught it lucky. This is a snapshot of the Sandhamn camera and it looks pretty nice. It's only in the mid 60's (or high teens if you roll Celsius), so a little chilly.
I found a great article on the archipelago from the American Way magazine, and the city of Stockholm has a page about the islands, and one on midsummer, too.