By maris and chin2

The seasons are popularly considered in some Western countries to start at the equinoxes and solstices, based on astronomical reckoning. Thus, in the Southern Hemisphere, based on astronomy, summer begins on the day of the December solstice and ends on the March equinox. When it is summer in the southern hemisphere it is winter in the northern hemisphere, and vice versa. But, because the seasonal lag is less than 1/10th of a year (except near large bodies of water), the meteorological start of the season, which is based on average temperature patterns, precedes by about three weeks the start of the astronomical season.[1] According to meteorologists, summer extends for the whole months of December, January and February in the southern hemisphere, and the whole months of June, July and August in the northern hemisphere. 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. From the astronomical perspective, spring days lengthen from equinox to solstice and summer days shorten from solstice to equinox, while meteorological summer encompasses the build-up to the longest day and a diminishing thereafter, so that summer has many more hours of daylight than spring.

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. Meanwhile, the astronomical definition is more frequently used in the United States.

In general, seasonal changes occur later in coastal regions, so countries lying near coastlines (except Ireland)[citation needed] experience the start of summer later than those lying inland.[citation needed] Elsewhere, however, the solstices and the equinoxes are taken to mark the mid-points, not the beginnings, of the seasons. In Chinese astronomy, for example, summer starts on or around May 5, with the jiéqì (solar term) known as Lixia (立夏), i.e. "establishment of summer", and it ends on or around August 6. An example of Western usage would be William Shakespeare's A Midsummer Night's Dream, where the play takes place over the shortest night of the year, which is the summer solstice.

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]

From a popular culture point of view, in the United States summer season, is often considered to begin at the Memorial Day weekend (the last Monday in May) and end at the Labor Day weekend (the first Monday in September). Likewise, another set of pop-cultural reference points for summer is the time when elementary and secondary schools close down for the "summer vacation". This period usually lasts from around early to mid June until around late August to early September, depending on where the school is located.

source: WIKI