Meteorologists generally define four seasons in many climatic areas: spring, summer, autumn (or fall) and winter. These are demarcated by the values of their average temperatures on a monthly basis, with each season lasting three months. The three warmest months are by definition summer, the three coldest months are winter, and the intervening gaps are spring and autumn. Spring, when defined in this manner, can start on different dates in different regions. In terms of complete months, in most North Temperate Zone locations, spring months are March, April and May, although differences exist from country to country. (Summer is June, July, August; autumn is September, October, November; winter is December, January, February). The vast majority of South Temperate Zone locations will have opposing seasons with spring in September, October and November. Swedish meteorologists define the beginning of spring as the first occasion on which the average daytime temperature exceeds zero degrees Celsius for seven consecutive days, thus the date varies with latitude and elevation.
Astronomical and solar reckoning
In the USA and some other regions in the Northern Hemisphere, the astronomical March equinox (around 21 March) is often taken to mark the first day of spring, and the Northern solstice is sometimes taken as the first day of summer (around 21 June in the Northern Hemisphere).
In South America, the Tupi-Guaraní calendar, from the former inhabitants of what is presently Brazil, Northern Argentina and Paraguay, counted 365 days, plus a fourth part of a day, needing no extra day every four years. The beginning of the solar year was marked by the rising of the M25 Constellation in the horizon, which occurs between June 5 and June 11 in this part of the world. For these native people, the four seasons were clearly identified by the solstices and equinoxes. The trajectory of the Sun throughout the year was divided into "The New Age" (Ara Pyau) and "The Old Age" (Ara Ymã). Ara Pyau was spring and summer, and Ara Ymã was autumn and winter. This calendar, which had no graphed or written form, marked activities such as hunting, fishing, planting, harvesting and religious rituals.
In East Asian Solar term, spring begins on 4 February and ends on 5 May. Similarly, according to the Celtic tradition, which is based solely on daylight and the strength of the noon sun, spring begins in early February (near Imbolc or Candlemas) and continues until early May (Beltane).
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