why is it summer in australia during christmas

Well, it's all about the Earth's tilt! The Earth's axis is tilted, which causes different parts of the planet to receive more or less sunlight depending on where it is in its orbit. So, during December, the southern hemisphere (which includes Australia) is tilted away from the sun, making it summer there. It's all a big coincidence that Christmas happens to fall during summer there, but it's a happy one!