The summer solstice
21 June sees the Sun reach its furthest northerly latitude - the summer solstice.
At 0424 UTC (~GMT = 0524 BST) on 21 June, the Sun reaches it furthest northerly latitude of 23º 26.1' N - the summer solstice for the northern hemisphere and start of astronomical summer. But, in meterorological terms, summer is the 3 moths of June, July and August - so had already started on 1 June.
The solstice moves on by ~6 hours each year (it will be 1007 UTC on 21 June 2018) due to the fact that the Earth takes ~365.25 days to circle the Sun; the inroduction of a leap year each 4 years (with some exceptions, including the turn of the centuary) keeps the date fairly consistent.
The solstice is also close to the 'Midsummer Day' of 24 June, which is one of the 4 Quarter Days in the UK Legal Calendar - the other Quarter Days are Lady Day (25 March), Michaelmas (29 September) and Christmas Day (25 December).
The Sun will pass back to the southern hemisphere at the autumnal equinox - 0802 UTC on 22 September - and, throughout Europe, Daylight Saving Time finishes at 0100 UTC on 30 October.
Further reading at the link below . .