From what I can see, there were only two Captain Marvel Christmas comic book covers and both of them were from the Captain Marvel Adventures series.
The first Christmas cover was Captain Marvel Adventures #19 which published on December 11, 1942. This cover featured Cap, Mary Marvel and Santa Claus! It featured the Christmas adventure, "The Phantom of the Department Store!"
The second Christmas cover was Captain Marvel Adventures #42 which published on November 24 1944. It had a beautiful full cover portrait of Cap surrounded by a Christmas wreath and a note stating, "Season's Greetings" and signed by Captain Marvel himself! This comic had the Christmas adventure "The Plot Against Christmas."
With Captain Marvel, I send you Seasons Greetings and wish you and your family a Merry Christmas and Happy Holidays!