If you have relatives living the other side of the world and you want to post their christmas cards by the cheapest method, i.e. by sea, then you will need to get them in the post by September. Also, if you have a relative serving with the forces abroard and you know they will be away on a tour of duty at christmas, you may just want to give them their cards before they leave for them to open chritsmas day. It's totally acceptable, we don't all hand deliver cards on christmas eve