I always found it strange that the day which marks the beginning of the season of summer is called “mid-summer”, which I understand would mean “middle of summer”. While midsummer is on the summer solstice (June 20–21), the actual middle of summer would be about August 6, no?
So why is the first day of summer called midsummer?
Definition 1. a. of summer in the OED is as follows:
a. The second and warmest season of the year, coming between spring and autumn; reckoned astronomically from the summer solstice (21 June) to the autumnal equinox (22 or 23 September); in popular use comprising in the northern hemisphere the period from mid-May to mid-August; also often, esp. as in (c) below, in contradistinction to winter, the warmer half of the year (cf. midsummer n.). (Often with initial capital.)
Using the popular definition of summer, the summer solstice occurs more or less in the middle of summer.