Since summer (and other seasons) can begin and end on different days each year on the astronomical calendar, it can be hard to compare seasons from year to year in this way. To standardize the length and start of the seasons, in meteorology, the 12 months of the year have been split up into four, 3-month periods. Therefore, in the meteorological calendar, summer includes the months of June, July and August - always starting on 1 June and ending on 31 August.