Since you asked I don't mind sharing - but I don't think much is different here except the weather LOL and please bear in mind I had assumed this was for youngsters!
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Judy (and I assume - students)
High on the list would be BBQs! It is so hot here around Christmas that some people substitute this for the traditional Northern hemisphere meal, and many cook the traditional ham and pork roasts on slow cooker BBQs outside.
And Christmas puddings made of icecream have always been big where I live. You mix dried and glace fruit into dark choc icecream with some extra cream (and maybe some liqueur) and freeze it in a big round pudding bowl to be cut into slices - just like a pudding. Yummy!!
Because we have the long school break over that time (just before Christmas to beginning of February) many people refer to the "Silly Season" as being from the first day of December to the Australia Day long weekend (26 January) - when all the partying and holidays occur. Govt Depts can be a bit erratic in responding to things around then because so many people are away on holidays, and most businesses close for at least two or three weeks. Not the shops, of course - they usually only close for a day or two. The big retail centres in the major cities trade all night for the week before Christmas. But I am from a small country town so we don't do that here.
Many of our Christmad cards feature Santa in boardshorts and thongs (what you call flip-flops I think) or surfing or laying around on the beach. I don't think there is anywhere in Oz that has snow at Christmas - except maybe the Antarctic Territory protectorate (which most people don't think of as part of Oz anyway!). If you want some Ozzie cards just let me know - happy to send over!
While many things are different, many more are the same. It is a time for getting together with family that you might not see all that often, and for exchanging gifts. Volunteer organisations put on Christmas dinners for thousands of people who are poor or homeless. Church attendance goes up for special services. Road and air traffic is high as people go on holidays. We still have Christmas trees and decorations, and some of us even spray fake snow on the windows (even though it's 100deg outside!). We play and sing most of the traditional carols, but do have a few that are just for Australia (like "Six White Boomers" that tells of Santa's sleigh being pulled through the Outback by six giant white kangaroos).
I worked almost every Christmas Day for my 20 years in the Royal Australian Air Force, and Christmas on a Base is very special. There is usually only a "skeleton" staff on duty, but we used to let them bring their families in for a big traditional lunch (all ranks together) and the kids would bring their toys in to show off. Then most people would go down and sit by the swimming pool to rest until they could go for a swim to cool off.
Because it is summer it is also bushfire time, and there are always fires somewhere. They can burn out thousands and thousands of acres and last for weeks. Most of our fire fighters are volunteers and they can be away for days or weeks fighting to protect properties. Some brigades will even drive many hundreds of miles to help in other states. And in my State we get the giant fire fighting helicopters from the USA to come out here for the summer months to help fight bush fires in the inacessible areas like National Parks. One of the 'copters is called "Elvis" and it always makes the evening news when Elvis arrives for the summer.
These are just some of my Christmas memories. Australia is a big place - bigger than the continental USA. So while the weather in Hobart Tasmania might be nice and mild (say 70deg) we could have 110deg in Adelaide, cold front through Perth (60deg) and cyclones in the northern tropical areas. We don't get many cyclones, but about 30years ago one wiped out Darwin (capital city of the Northern Territory) right around Christmas.
We are also very multi-cultural. We have a lot of different races and religions here, but most of us will get together and celebrate around this time of year. I have been invited to a Christmas Dinner with some friends who are Buddhist vegetarians, so that will be very interesting this year. They are from Sri Lanka, and they believe that since they have come here to live they should celebrate our festivals with us, so we will invite them to join us for Easter.
So Australia has as many different Christmases as the many different States over there do. It's just that most of ours relate to Summer things instead of the traditional Northern Hemisphere Winter things.
I wish you joy and peace for the coming year - however and whatever you celebrate.
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So you see Tina - it's not so very different here!