In the Northern Hemisphere, the year is divided into four seasons. Each season begins at a solstice or an equinox.
In the Southern Hemisphere, the dates (and the seasons) are reversed. The summer solstice (still the longest day of the year) falls around December 21, and the winter solstice is around June 21. So when it's summer in North America, it's winter in South America (and vice versa).
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