The Sun reaches its highest northerly latitude on 20 June.
The Sun will be at its highest latitude of ~23.4ºN at 2143 UTC (2243 BST) on 20 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 on 22 September - the autumnal equinox and end of astronomical summer. It will start its northerly journey again at the winter solstice on 21 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 - as it was this year.
And clocks in most of Europe will remain on 'summertime' (EST and BST) until 25 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. Whether UK sticks to this schedule remains to be seen.