In the northern hemisphere, the longest day of the year (around June 22) occurs when the sun is farthest north and marks the official first day of summer, although weather wise we consider June 1st as the start of a three month summer period which ends on August 31st. The summer solstice marks the period when the length of time between sunrise and sunset is at a maximum for the year.
 
The area that sees the very longest period of possible daylight (and therefore sunlight) in the UK is in the far northern isles of Scotland near the town of
Here the sun rises at about 0330 on June 21st and sets at about 2241 BST, giving -in theory- the possibility of around 19 hours sunshine. In practice such amounts are rarely achieved.

However some months can be extremely sunny in June. For example, Pendennis Castle, Falmouth, recorded 381.6 hours, the highest monthly sunshine record for this century, an average of 12.7 hours per day.