when is midsummers day?

It varies from place to place, culture to culture, but it's generally on or near the summer solstice, which admittedly is the start of summer, making the name 'Midsummer' a misnomer, but then we're used to misnomers, like 'midnight' isn't in the middle but at the very end of night and the very start of the wee hours of the morning.

Midsummer's Day is the summer solstice. The  solstice varies from year to year and the various time zones affect the date of the solstice. The June solstice this year will be at 3.31am UTC on Monday 21 June. That means that in the Americas it will be on the 20th June while Europe, Africa and all points east will be on the 21st.

In the southern hemisphere, the summer solstice is at 4.58pm UTC on 21 December which means it will be on 22 December in eastern Asia and Australia.

Some parts of the world start their seasons on the solstices and equinoxes which makes Midsummer's Day the first day of summer. Other countries, like Australia, start on the 1st of the month so summer starts on 1 December. This makes quarterly statistics very easy. As the choice of the start of seasons is entirely human artefact, there is no reason the rest of the world shouldn't do the same.

20th June in the US and 24th June in UK.