When it’s summer in New York, it’s winter in Sydney. When kids in the US are starting school in early September, kids in South Africa are starting their summer break. The Northern and Southern Hemispheres experience opposite seasons because of how Earth’s tilt works.
Earth tilts 23.5 degrees on its axis as it orbits the Sun. For half the year, the Northern Hemisphere is angled toward the Sun and gets more direct sunlight - its summer, and the Southern Hemisphere’s winter. For the other half, the Southern Hemisphere is angled toward the Sun and gets summer, while the Northern Hemisphere has winter. The seasons in any pair of opposite-hemisphere countries are 6 months out of sync.
This means Christmas in Australia is during summer - temperatures regularly hit the 90s°F, beaches are packed, and a Christmas Day swim is a national tradition. New Year’s Day in South Africa is summer too. Meanwhile, August in Argentina is the dead of winter, with snow in the south. South American ski resorts run their seasons June through September while North American resorts are closed for summer.
