I think that this is a personal decision and would encourage you to do what feels right to you whether this is to send cards or not. When I was in this situation some years ago, I did not send any cards the first Christmas after my bereavement. This was my personal decision, I was not aware of whether there was a 'custom' or not. I received cards which I appreciated. I always make a special effort towards somebody who is going through that 'first Christmas' to send them a special card, and write a note to let them know that I am thinking of them. It is a milestone for the bereaved family and as there is such an emphasis on Christmas being a family time it can be very difficult.