So I took a look on the internet for historical data. First I had to calculate the actual date of the midsummer holiday, as it's set as the Saturday that falls within June 20-26. (It was on my birthday in 2000 and 2006; this year, it's early; the 20th.)
Then a look on www.wunderground.com gave me the Stockholm observations on the appointed Saturdays this decade. The results were:
- Average high 18.6C/66F
- Average low: 12.6C/55F
- Rain: on 5 of the 9 dates
NB: These are Stockholm observations and the weather is different in the archipelago, although I think the general trend is correct.