He-man and She-Ra Christmas Special (1985)

Honestly I can't remember much about this because I was laughing hysterically.  We may have been overtired as this was the last thing we watched for the night, but oh my god it almost killed us!

All I can remember is that screw up Orko mistakenly goes to Earth, meets two kids, and brings them back to Eternia.  The kids teach everyone about Christmas and Skeletor's heart grows a smidge, he sheds a tear and does something kind.  There are five million new characters brought in every other minute, but it's so ridiculous that you won't care.

The best summation I can think of is here - The Greatest Christmas Story Ever Told