From an astronomical view, the equinoxes and solstices would be the middle of the respective seasons, but sometimes astronomical summer is defined as starting at the solstice, the time of maximal insolation, or on the traditional date of June 21. A variable seasonal lag means that the meteorological center of the season, which is based on average temperature patterns, occurs several weeks after the time of maximal insolation. The meteorological convention is to define summer as comprising the months of June, July, and August in the northern hemisphere and the months of December, January, and February in the southern hemisphere. Under meteorological definitions, all seasons are arbitrarily set to start at the beginning of a calendar month and end at the end of a month. This meteorological definition of summer also aligns with the commonly viewed notion of summer as the season with the longest (and warmest) days of the year, in which daylight predominates. The meteorological reckoning of seasons is used in Australia, Austria, Denmark, the former Soviet Union and Japan. It is also used by many in the United Kingdom. In Ireland, the summer months according to the national meteorological service, Met Éireann, are June, July and August. 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.
A - First Six-Week Session: May 21 - June 29
B - Ten-Week Session: June 4 - August 10
C - Eight-Week Session: June 18 - August 10
D - Six-Week Session: July 2 - August 10
E - Three-Week Session: July 23 - August 10