Christmas cards, as well as other holiday and special occasion cards, can be a great wealth of information. If you are lucky, maybe someone in your family has stashed away these cards. Most likely these cards will assist you in researching those from the 20th century
Christmas cards provide the names of family and friends, the envelopes provide addresses and in some cases the cards may provide a newsy letter that can lead to more clues to finding your family.
Genealogy isn't just about researching the dead. I have got into the habit of saving my current Christmas cards, both an example of ones I send out and the ones I receive. Why? Because those signatures of granparents, aunts, uncles and far off cousins might be meaningful to my children when they grow up and might provide them clues. One of the cards I cherish is one from an aunt who died several years ago. She always signed her familiy's Christmas card in the shape of a Christmas tree. One of my memories of Christmas was receiving that familiy's card and that distinctive signature.
One more story involving Christmas cards. After a cousin died and I was taking care of her estate, I used Christmas cards and her Christmas address labels to figure out family members that I should contact. She was from a line where I did not personally know many of the family members. So by using her cards and addresses I was able to find family members to contact and let know of her passing. I was also able to fill in some family group sheets.
Christmas cards provide just one more piece to the family history puzzle.