Message:
Why do people living in Australia celebrate Christmas in the summer?

Simple really! It's because it is located in the southern hemisphere!
Their summer is the northern hemisphere(NH)winter time while NH winter will be their summer!
The position of the earth is made in such a way that there is a tilt in the axis of approximately 23 degrees. Due to this tilt the NH will experience a winter during December time while the south experiences a summer during December time. While Christmas is popularly shown as a snowy and winter kind of festival, this cannot be applied to Australia or even a matter of fact Brazil which has a huge Christian population.