SUMMER STARTS WHEN? – The astronomer would reply, “At the solstice.” The meteorologist might respond, “On June 1st.” The student could retort, “When school’s out.” The dogs are tired of rain and do not care what the calendar says. They are declaring the arrival of “summer”

The solstice is the beginning of the astronomical summer that has been celebrated since ancient times as the longest day of the year, whereas Midsummer now refers to numerous celebrations that are held over the solstice period, between June 19 and June 24, with both pagan and Christian origins.

Seasons in the Southern Hemisphere are opposite to those in the Northern Hemisphere. For example, under the definition of astronomical seasons, the June solstice marks the start of summer in the Northern Hemisphere, but it is the start of winter in the Southern Hemisphere. The same rule applies for the other seasons. The meteorological seasons in the Southern Hemisphere are also opposite to those in the Northern Hemisphere.

The question which definition to use divides countries and regions around the world. For example, Australia and New Zealand use the meteorological definition, so spring begins on September 1 each year. In many other countries, both definitions are used, depending on the context.
In Ireland, St Brigid’s Day on February 1 is often thought to mark the beginning of spring in the ancient Celtic calendar system. Some cultures, especially those in South Asia have calendars that divide the year into six seasons, instead of the four that most of us are familiar with.
In Finland and Sweden, the dates of the seasons are not based on the calendar at all, but on temperatures. This means that the seasons within each county start and end on different dates, depending on the regions and their climate. In Iceland, the first day of summer, a national holiday, falls on the first Thursday after April 18.
With celebrations originating in ancient times as a festival for the summer solstice, this could be where the name comes from. Bonfires were lit up to ward off evil spirits as the sun turned southward. If you only had the sun to measure time and dictate your hours of light, the longest day would feel like a mid point before you make the descent back into winter.
SOURCE: B.B.C; C.N.N.