"At midsummer" would make sense to me. I don't think too many BE users would say "on Midsummer."
"At midsummer" would refer to the time of year. The capitalisation of the quoted answer, shows that this is referring to a specific day to celebrate the summer solstice. "On" would be appropriate for a particular day, just as we would say "at Christmas(time)" but "on Christmas Day".
Personally I have never heard a specific day called simply "Midsummer" in English. "Midsummer Day" (or "Midsummer's Day") is the 24th of June, one of the four "quarter-days".
I would definitely say 'on' in the same way that one might say "I ate a lot of cake on saturday" or "they ate turkey on Christmas day" at or in sounds weird in this sentence.
Midsummer is a "holiday" is it not? It isn't the set of weeks durring the months of july and august correct?
Yes, it's a holiday around the summer solstice. This year it will be June 19-20th.
Dear Arnauti nobody answered to far up question..why... lots of...is not accepted
"They eat much fish during midsummer," was counted as wrong. Could anyone tell me why that is?