Russian Orthodox Christmas is celebrated in January, mostly by those who are devoutly religious. Since Russia was primarily secular during the Soviet period, the major holiday celebration in Russia is actually New Year's Day, which has developed a romance of its own.
This is the day for gluttonous tendencies which Americans reserve for Thanksgiving, and for the adults, centers on elaborate food and drink. At the stroke of midnight the champagne is uncorked, the TV is turned on and everyone listens to the Kremlin bells while toasting with the words "S novom godom" which mean Happy New Year.
After the President of Russia gives an Official New Year's greeting to the people over the airwaves the party really gets started. Some sing and dance in their apartments while others go out into the streets for more festivities which include impromptu accordion playing, sparklers, fireworks and more singing and dancing on street corners. Most cities or towns which have snow build ice sculptures, and that tends to be the area where the public gathers as the night comes to life.
For children the basic ingredients are Grandfather Frost (Santa) and the Snow Maiden (his lovely assistant) who distribute presents which are placed under the New Year's tree during the night of December 31-January 1.
Grandfather Frost was originally an old Russian folk tale and personified the cold. He is generally taller and thinner than the traditional Santa Claus in the United States and has more clothing options (he sometimes wears blue and white). The "yolka" (New Year's Tree) is often decorated with small edibles.
Since the Russian mail system is not very reliable, they tend not to send greeting cards. If, however, you are able to correspond with your Russian colleague by e-mail, it would be very appropriate to wish them a Joyous and Happy New Year.