As you may already know, it is indeed winter in U.S., but in places below the equator, summer is starting to heat up. That is why you typically hear about Australian wildfires during the holidays. This is a result of the tilt of Earth’s Axis being at an angle. During what is summer for the Northern Hemisphere, the Southern half of the Earth is put closer to the sun, leading to higher temperatures and longer days, as the inverse is also true for summer in the Northern half of the world. Thus, the Southern Hemisphere has the Winter Solstice in June around the 20th, while the Summer Solstice occurs closer to Christmas, near the 21st.