Depending upon the event or occasion for which you are sending a card, the message may be specific to a particular day, or the sentiment may be that of a sympathy note. If you are unsure which specific holiday the person celebrates, use a generic phrase such as "Happy Holidays."
When you don't personally know the person you are addressing the card to, it is best to be as generic as possible. For sympathy cards, you can write "Sorry for your loss," "with sympathy," or "sending prayers and positive thoughts for you and your loved ones." You can sign the card with your first and last name and your company, if you feel that will better establish your identity.