We are celebrating something like Midsummer in Latvia as well (just across the Baltic sea from Sweden). It is the day, in which the time of daytime is the longest.

Ahhh, much thanks.

Your explanation makes more sense... If you take summer in countries in north of equador, the period that goes from summer solsticium to the autumn equinocium, obviously the middle of summer is in august... This Midsommer must come from the fact that spring and summer are the warm seasons, when the day is longer then night... And the summer solsticium is right in the middle of it... It is just a theory, although

Well, if that proper noun is truly unknown in English, you don't translate it at all. It would be capitalized and spelled in English as "Midsommar". This is the same as you might refer to other things that don't have local anglicized names like perhaps "Ramadan" you don't translate into English before writing.
That being said, the holiday was co-opted long ago by (some) Christian sects to be labeled Saint John's Day (and Saint John's Eve) for the nativity of their martyr in the same way the nativity of a later martyr relabeled the traditional winter solstice holiday as Christmas, so it is actually not completly unknown to many English speakers and I think it is celebrated nationally in some Anglo countries. I think it's sometimes known as capitalized proper noun "Midsummer" (and Midsummer's Eve) Even your suggestion might less commonly be correct, the holiday being sometimes known as capitalized proper noun "Summer Solstice" (and Summer Solstice Eve), not to be confused with the certainly incorrect translation "the summer solstice" being the actual astronomical solstice of summer.

Shakespeare would presumably disagree with your contention that Midsummer isn't used in English, given the word is literally in the title of one of his most famous comedies. Without a hyphen space, midsummer's day is definitely the 21st June.

I never noticed that before reading this discussion.

To be fair, this is rather vague as a comment. I live in Sweden and when people here talk about midsommar its usually to do with the holiday period and not referring to the solstice day.
To translate this it could be that you are translating when the swedish holiday is as equally as when you are saying the solstice is. Sorry, just think this is too vague to not allow both translations.
Just my 2p :)

Thank you!

I don't think the difference is as strong in English since it's not really a holiday in the English speaking world. The spelling wouldn't have made me think one thing or the other in English. I think you'd get your answer from the context more than anything. If someone says I'll be back in mid summer, that's clearly an adverbial phase due to the rest of the context. I think it's also just not said that often. I think 'in the middle of the summer' would be a more typical way of talking about it in English, which is why I was surprised to get it wrong rather than the spaces being extremely meaninful. Duo is also just so lenient about typos and extra spaces that I didn't think it would care (which I generally appreciate - my keyboard is getting old and spacing related typos abound, especially on timed practices).

Midsummer in English means the middle of summer. There is no "Midsummer" holiday in the English speaking world. I didn't realize this was referring to a specific holiday until I checked the comment section. I think this will likely confuse other English speakers as well since it is a generic term for us.

Yes, and one would use lower case for a generic mid-summer but uppercase to indicate a holiday named Midsummer. The reader need not be familiar with the holiday to notice the difference.