Christmas wasn't publicly celebrated in Soviet times, only in church on January 6th. They still had a Santa like figure, Father Frost, whose sidekick is the Snow Maiden ,Snegurochka
The greetings on these cards is not "Merry Christmas" but, more or less, "Happy New Year". The tree depicted on some is a "New Year's Tree"
https://savedfromthepaperdrive.blogspot.com/2011/12/soviet-space-christmas-cards.html