Because in Australia, its summer during december and so christmas is celebrated during summer.

Introduction:
Christmas is celebrated on December 25th in many countries around the world. However, in Australia, Christmas falls during the summer season. This is due to the geographical location of Australia and the motion of the Earth.

Reasons for Celebrating Christmas in Summer:
There are several factors that contribute to the celebration of Christmas in summer in Australia.

1. Geographical Location:
Australia is located in the southern hemisphere, which means that it experiences the opposite seasons compared to countries in the northern hemisphere. While countries in the northern hemisphere, such as the United States and the United Kingdom, experience winter during December, Australia experiences summer.

2. Tilt of the Earth's Axis:
The Earth's axis is tilted at an angle of approximately 23.5 degrees. This tilt is responsible for the changing seasons throughout the year. During December, the northern hemisphere is tilted away from the Sun, resulting in winter. On the other hand, the southern hemisphere is tilted towards the Sun, leading to summer.

3. Revolution of the Earth:
The Earth revolves around the Sun in an elliptical orbit. This revolution takes approximately 365.25 days to complete, resulting in the length of a year. As the Earth revolves around the Sun, different parts of the world receive varying amounts of sunlight and experience different seasons.

4. Christmas Traditions:
Despite the difference in climate, Australians still celebrate Christmas with traditional festivities. However, they adapt their celebrations to suit the summer season. For example, instead of a traditional Christmas dinner with hot roasted meats and heavy desserts, Australians often have barbecues and enjoy outdoor activities. Many people also head to the beach to celebrate Christmas and enjoy the warm weather.

Conclusion:
In Australia, Christmas is celebrated during the summer due to the country's geographical location in the southern hemisphere and the tilt and revolution of the Earth. Despite the differences in climate, Australians still embrace the festive spirit of Christmas and adapt their celebrations to suit the warm weather.