Following the tradition of our parents, from the time we married, Christmas cards were sent out to many family and friends. That custom slowly dwindled (unfortunately!), and was replaced by the Christmas email letter. There may have been some before 1968, but that's the earliest on record.
They provide a useful summary of events of the past year, sometimes with pictures.
Some additional ad hoc letters are also included if they provide useful contemporary information on events at that time.