Summer in December
While much of the world is bundled up for winter in December, Australia is enjoying summer. December, January, and February are the summer months across Australia. This means Australians often spend Christmas at the beach, in shorts and sunshine, rather than in snow.
Why the Seasons Are Flipped
The reason comes down to which half of the planet Australia sits in. Australia is in the Southern Hemisphere, the bottom half of the globe. The Southern Hemisphere has its seasons reversed compared to the Northern Hemisphere, where countries like the United States and most of Europe are found. When the northern half of the world is tilted away from the Sun and experiencing winter, the southern half is tilted toward the Sun and experiencing summer.
A Year Turned Upside Down
This flip applies to the whole calendar, not just December. When the Northern Hemisphere has spring, Australia has autumn. When the north has autumn, Australia has spring. For travellers, it can be a strange but pleasant surprise: fly far enough south and you can step out of winter and straight into summer.
Source
This article was written using information from Wikipedia.