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 Sun.
Hot UK summer more likely this year, Met Office says
When does summer start? - Met Office
2023 in weather Official blog of the Met Office news team
Met Office gives verdict on whether UK will hit 40C this summer
Summer Herwarth Facebook, Instagram & Twitter on PeekYou
Met Office weather forecast declares Indian summer could continue
Met Office issues weather warnings over extreme temperatures in coming days
The hottest day in Weymouth this week according to the Met Office
Summer - Met Office
Deep Dive 30/05/2023 – Summer outlook special – Met Office weekly weather forecast UK - video Dailymotion
Deep Dive 25/07/2023 – Subtle shifts in the jet stream - Met