I posted several '80s-era Christmas songs. Here's some from the '70s.
Elton John "Step Into Christmas" This one's a well-remembered classic. Very catchy and inviting. It really does make one want to step into the holidays.
John Denver and The Muppets "The Twelve Days of Christmas" This one is so wonderfully '70s, and always puts a smile on my face.
The Carpenters "Merry Christmas Darling" A happy memory from a classic '70s duo. Very traditional and homey, and Karen with her wonderful voice.
Slade "Merry Xmas Everyone" A fun Christmas song from the Glam part of the '70s. This is more well known in the UK than in the US.
Greg Lake "I Believe In Father Christmas" And from the Prog side of the '70s, this song which is an interesting combination of Christmas song and protest song. One does not have to agree with all of the lyrics to find it compelling.