The Sun reaches its highest northerly latitude on 21 June.
The Sun will be at its highest latitude of ~23.4ºN at 1554 UTC (1654 BST) on 21 June - the summer solstice. This is the start of astronomical summer although, in meteorological terms, summer is defined as the months of June, July and August.
The Sun will cross the celestial equator to re-enter the southern hemisphere at 0750 UTC on 23 September - the autumnal equinox and end of astronomical summer. It will start its northerly journey again at the winter solstice - 0419 UTC on 22 December. As the Earth takes ~365.25 days to orbit the Sun, these times become around 6 hours later each year; hence, to keep dates and seasons consistent, a leap day is inserted every 4 years.
And clocks in most of Europe will remain on 'summertime' (EST and BST) until 27 October. The EU stipulated in 2002 that all member states should adjust their clocks on the last Sundays in March and October, the change to occur at 0100 UTC.