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:
1. 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.
The word midsummer comes to us from Old English, and it has a Dutch cognate midzomer, and Scandinavian cognates (e.g. midsommar in Swedish), so it may even come from an older Germanic language. Both the old Anglo-Saxon calendar and the old Icelandic calendar had two seasons, summer and winter. For these calendars, "Midsummer's Day" would have fallen near the middle of summer (probably not the exact middle ... summer started in mid-April in the old Icelandic calendar, and on a full moon in the old Anglo-Saxon calendar).
The Anglo-Saxon calendar also explains why summer and winter are words which have roots in Proto-Germanic, while fall and spring were not used for the seasons until Middle English, and autumn is originally a Latin word.
Wikipedia has an extensive definition of solstice. There is a section for solstice celebrations, such as "midsummer."
This excerpt may help solve the puzzle of the relationship between the midsummer and the summer solstice.
In some languages they [the solstices] are considered to start or separate the seasons; in others they are considered to be centre points (in England, in the Northern Hemisphere, for example, the period around the northern solstice is known as midsummer, and Midsummer's Day is 24 June, about three days after the solstice itself).
It's an astronomic question vs. a cultural & meteorologic one.
Astronomically, the summer solstice is considered to be "mid-summer", because it is technically the longest day of the year. However, using this definition would place the beginning of summer sometime in early-May, and the end of summer in early-August. For most people, this simply does not make sense, due to our cultural conceptions of the idea of "summer" as the time of year when things are warm and sunny. The given astronomical definition just does not line up with this idea at all!
Meterologically (weather-wise) the summer-solstice is the time of year when things are just STARTING to warm up, so it makes sense, from that cultural perspective, to refer to the solstice as "the first day of summer", rather than "mid-summer".
Simply put, our modern calendars and media have gotten it wrong. Somewhere along the line of history, perhaps as we became less rural and more urban as a society, the accurate meanings of Midsummer and Midwinter have been lost. One has only to consider the fact that the Solstices mark the MID POINT of the seasons via the course of the Sun. Basing calculations on four seasons, Summer would begin around the first of May, hence the old May Day celebrations, marking that very thing. This is the period when the planting is completed and the Earth is again filled with growing foliage. The Sun is moving toward its' annual apex or high point, which falls at Midsummer around the 20th of June, marking the longest day of the year. After that point, the Suns' energy begins to wane as it reverses its' course back toward Winter and the days grow shorter. (Hence, to say Midsummer is the beginning of Summer is contradictory.) The Autumnal Equinox marks the manifestation of Fall, when the hours of day and night are equal, though it could be said Fall begins around the first of August, when the nights start getting cooler and the foliage begins to dry up. The Harvest season also marks the beginning of the Autumn Tides. Winter as a season begins on the 1st of November, which is why 31 October was thought of as a time of death (the "dying" of the Earth until the next Spring), and came to be associated with celebrations such as Samhain, and later Hallowe'en. Around the 20th of December, we reach Midwinter, or the middle of the Winter season, at which point we experience the shortest day of the year and the Sun then begins to wax in energy as the hours of daylight again grow gradually longer. This explains the Midwinter celebrations (Yule, Saturnalia, etc.) which later became Christmas, as it marks the return of the powers of light in the midst of the deepest darkness. Winter begins to lose its' grip around Imbolc (now Groundhog Day in the USA), when the first stirrings of life begin to reappear, though there are technically six weeks of Winter remaining. The Spring Equinox ushers in the astronomical Spring season, as the hours of daylight and darkness are again in balance, as indicated by the old Spring celebrations which led to our modern Easter (a term based on the name for an old Celtic/Anglo Goddess of Spring). And we soon find ourselves back at May Day, when Summer actually begins.
Of course, in the Southern Hemisphere this is all reversed, as their Midsummer is in December and their Midwinter in June.
In our modern world, we tend to focus on the seasons as they apply to us or our particular region, and not on the astronomical reality which few pay attention to these days. Schools generally let out around the first of June, and many families take vacations during the three months we've come to think of as Summer, so it's understandable as to why we got so confused as to Midsummer and the beginning of Summer. Sadly, not many seem interested in correcting this error, as I have contacted several media sources (including the Old Farmers' Almanac), all to no avail.
In German, 'mit' means 'with' and in the germanic old English 'Mit Sommer' meant 'With Summer'. The old English song of the Solstice is 'Sommer is icommin in' and not, "Sommer is y haf igonne'. :)
