Thanks for starting this thread, Hanna!

The "Art Nouveau" card is interesting -- it says "Merry Christmas and a Happy Upcoming/Approaching New Year". Nowadays, as far as I know, Russian Orthodox Christians use the Old Style (Julian) calendar for the date of Christmas, but New Style (Gregorian calendar) for the date of the New Year -- thus, Рождество "наступает" после Нового года (Christmas "approaches" after New Year's Day.) I would've guessed that this was an Ivan Bilibin design, but the initials at the bottom are В.З.

The art on the "Red soldier" card is interesting to me, too:

(1) The Star of Bethlehem (Вифлиемская звезда) is clearly six-pointed, and thus it looks like the "Star of David". Which is appropriate, in a way, because Jesus was Jewish, although Judaism did not begin using the six-pointed star until medieval times. But it surprised me a little to see the star as six-pointed, because in American culture and Christmas-art, the Star of Bethlehem is very "standardized", and almost without exception is four-pointed, usually the two horizontal points shorter than the two vertical points (thus, the star visually resembles the Christian cross).

(2) The soldier is clearly a Communist, yet the Russian very specifically says "Happy Nativity of Christ", instead of being shortened to "Happy Nativity." (Of course, "Nativity"/Рождество are religious terms, also, but mentioning Christ's name makes it more OBVIOUSLY religious.)

(3) What's that "Oriental"-style inscription in the lower left? It looks like it could be Korean -- or it might be Cyrillic letters written in a pseudo-Chinese style, I'm not sure.