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Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Christmas. How did it originate and how did it come to be what it is today?

I have heard that it originated from a Pagan holiday, but don't know much about the topic. Can somebody please help me understand how it shaped into what it now is?
gpt
Many cultures have 'winter solstice' festivals in and around the 25th of December. This was because days were short and dark, as little as 5 hours between sun rise and sun set in some northern Scandinavian capitals (Oslo etc)

So the cheer themselves up around the time of the shortest say they would get together, have a party, exchange gifts, etc.

Later Christianity became the dominant religion and pagan festivals could no longer be tolerated, so a story was created about the birth of Christ. (even though there is major dispute over this date)

However many of the trappings of the pagan festivals can still be seen. Evergreen tree to remember even in deep winter plants can survive. Mistletoe and holly to ward of spirits. And so on.

Each of the main countries that had these festivals (Gemany, Denmark, the UK, Norway, etc.) had a 'character' to go along with them, Saint Nicholas, sinterklaas, Father Christmas, and so on.

When immigrants went to the US there needed to be someway of amalgamating these disparate elements. So gradually over time, the elements of 'German Christmas' 'British Christmas' and so on all blended together into the modern day incarnation, and Santa Claus was born to play the character in this new version. Shoes filled with candy were replaced with stockings over the fire place, candles mounted on ever green trees were replaced with the far more sensible electric lights.

You can actually still see elements of the old characters in Santa Claus, both because of the name (Santa Claus is very close to Sinterklauss) and also because if you call him 'St. Nick' or 'Father Christmas' people know who you are talking about despite these figures being different and pre-dating him.

Eventually the US became the dominant cultural force and exported its version of Christmas back to Europe.