It is spring in Australia

Some countries start each season at a solstice or an equinox; counted that way, summer here would begin around 22 December. The Bureau of Meteorology uses 1 December, 1 March, 1 June and 1 September instead, because that matches Australian temperatures better. Across much of southern Australia the hottest and coldest weeks of the year arrive weeks after the longest and shortest days, because the surrounding oceans are slow to warm and slow to cool.

So "the first day of summer" in Australia is 1 December, every year, and it never moves. Only the weekday it lands on changes.

In the far north the temperature barely changes across the year, so the four-season model does not describe it. The Bureau divides the tropical year by rain instead: the wet season, October to April, and the dry season, May to September. When the rain actually arrives varies from year to year and place to place.

Indigenous Australians have kept their own local calendars for thousands of years, with three, five or six seasons read from the weather and from the life cycles of plants and animals. The Bureau publishes some of them on its Indigenous weather knowledge pages.