In a study that we carried out focussing on buying habits in 2018, we found that the highest number of Christmas Card printing orders we received in any one day was on Tuesday 5th December. Aiming to have your cards designed and printed by early December then allows for them to be sent soon afterwards, arriving with the recipient 2-3 weeks before the big day.
This will be early enough to avoid any potential delays that can be caused when the postal service gets incredibly busy, but not too early to seem unusual to the person receiving it. The Royal Mail have a page on their website dedicated to Christmas which includes useful information such as last posting dates.