According to meteorologists, summer extends for the whole months of June, July, and August in the northern hemisphere and the whole months of December, January, and February in the southern hemisphere. From an astronomical view, the equinoxes and solstices would be the middle of the respective seasons, but a variable seasonal lag means that the meteorological start of the season, which is based on average temperature patterns, occurs several weeks later than the start of the astronomical season.
However, according to the Irish Calendar summer begins on 1 May and ends on 1 August. School textbooks in Ireland follow the cultural norm of summer commencing on 1 May rather than the meteorological definition of 1 June.
Summer is the warmest of the four temperate seasons, falling between spring and autumn.
Source: Wikipedia