Christmas is the time to spread love, joy and happiness. Christmas letters could be of two kinds, personal correspondence and professional correspondence. Personal correspondence would include greetings to family, friends or relatives. Professional correspondence on this occasion could be a greeting letter by the organization or employer to its employees, client organization, to customers etc.
Since the occasion is of festivity an informal tone even to professional correspondence may be allowed. For example the stationary or the template used could have a theme Christmas. Personal letters on this occasion can be written according to the audience. The idea is that the letter should convey love, warmth and genuineness to the addressee.
Professional letters too should be written with the audience in mind. For example if it is for the employees it could start with warm regards, wishing a merry Christmas to him and his family, if there is a bonus check included then a mention of it should also be made. It is also an opportunity to acknowledge his dedication and loyalty towards the organization.
Letters to business associates could be written on same lines. What could make it more effective is if you make it a personal one. Incase it’s a group mail then a personal at bottom may do the trick too.