Summer is one of the four temperate seasons. Summer marks the warmest time of year with the longest days.The seasons are popularly considered in some Western countries to start at the equinoxes and solstices, based on astronomical reckoning. Thus, in the Northern hemisphere, based on astronomy, Summer begins on the day of the June solstice and ends on the September equinox. When it is Summer in the northern hemisphere it is winter in the southern hemisphere, and vice versa.Today, the meteorological reckoning of the seasons is used in Australia, Denmark and the former USSR; it is also used by many people in the United Kingdom, where Summer is thought of as extending from mid-May to mid-August. In Ireland, Summer begins on 1 May and ends 31 July, by the Irish calendar. Meanwhile, the astronomical definition is more frequently used in the United States.In southern and southeast Asia, where the monsoon occurs, Summer is more generally defined as lasting from March to May/early June, their warmest time of the year, ending with the onset of the monsoon rains.[citation needed]