I agree with those people who say this is a bad idea. There is just too much room for it to be seen as a negative. For example, some people don't even celebrate Christmas or Hannukah and this would be an insult. Even if they are Christian, they may despise the commercial nature of Christmas and be offended. Think about it, no one mails Christmas cards to people they don't really know personally except businesses. You mail them to friends and family. The PDs are neither of those. Just send a thank you card and call it a day.
FYI: To the poster who advises "you need to separate yourself" I would say, I hope someone warned you about the dangers of standing out too much. For example, I was told it is better to have a "somewhat" boring personal statement because being too over the top can hurt you. You want your academic record and accomplishments to stand out. Medicine is still a conservative field --- if you don't believe me, notice that all applicants are very similarly dressed (suits are black, gray, brown, blue, occasionally ivory, sometimes pin striped). The most "unique" item one may wear may be a snazzy tie (men) or bright colored blouse (women). Usually men remove earrings and women have very tiny nose earrings if they have nose jewelry.
