First Day Of Summer 2025 Uk Time. The period when the clocks are 1 hour ahead is called british summer time (bst). Summer 2025 starts on friday, june 20th in the northern hemisphere, with the first day of summer being the longest daylight day of the year with the earth's axis having reached its furthest tilt and begins to change.

Usually, when we talk about the first day of summer, we are referring to the astronomical summer which is defined by the earth’s axis and orbit around. The first day of summer is 20 june 2025.