Mid-'summer'

Today is the longest day of the year but midsummer didn't seem quite the right term for this morning. It was grey, cool and drizzling as I cycled in to work and I arrived with wet socks and soggy butt. Delightful. Why is it called "midsummer" when most of summer comes well after it? Why not "mid-year"?

Soggy Kruununhaka (downtown Helsinki)