December 1st 2009...the first day of Summer

n countries such as the UK, Denmark, Ireland and Australia the seasons are simply decided according to the Roman calendar, with the seasons decided by the hottest / coldest quarters of the year and beginning on the first of the month.
So Autumn - March 1st
Winter - June 1st
Spring - September 1st
Summer December 1st
and reverse for the opposite hemisphere.

For countries such as the United States, the seasons are determined Astrologically - and begin at the solstice and equinoxes. So spring begins when the days hit equal length then get longer, autumn when they hit equal length then get shorter etc.
Thus spring - March 19 - 22 - the spring equinox
Summer - 19 - 23 June - the summer solstice
Autumn - 21 - 24 Sept - the autumn equinox
Winter 20 - 23 Dec - the winter solstice
and vice versa for the opposite hemispheres.

Since the idea of four seasons was imposed on the rest of the world by northern Europeans and north Americans, there has been dispute about the start of the seasons. Most countries that subscribe to the four season idiocy start them on the solstices and equinoxes. Australia starts them on the first of the month.

My investigations have found a possible reason for this although it could well be apocryphal. The NSW Corps changed from its summer to winter uniform on the first of March and from its winter to its summer uniform on the first of September. The starting dates for the other seasons followed this.

It is much easier for record keeping if the seasons start on the first of the month rather than an arbitrary date between the 20th and 23rd. The Bureau of Meteorology certainly likes starting the seasons on the first.

Seasons were named and identified long before anyone knew that the Earth revolved on its axis, that the axis was tilted or that the earth orbioted the sun. They were identified from the behaviour of animals and plants and from the weather. Northern Europe settled on four seasons. In Australia, there are five named and identified seasons in the Kimberley, six in the Darwin/Kakadu area and the Wik people of Cape York have eight. Each of these is just as valid as the northern European four.

Information found on Yahoo 7 answers