The latest I've ever started was 3.20p.m., Christmas Eve - and that included all of my Christmas cards, some of which were for posting to Australia! Normally I start around 20th December. My Christmas decorations (usually a few bits of tinsel around the computer monitor and some flashing lights around the telly) are put in place first thing on Christmas morning and removed at the end of Boxing Day.
One of my colleagues at work is just the opposite. She can always tell you the exact number of days to Christmas (even in mid-January). I'm sure she will have finished most of her Christmas shopping by now (but she'll keep on buying 'little things' right up until the last minute. She starts putting Christmas decorations everywhere before the end of December and, unsurprisingly, her mobile ringtone is "We wish you a merry Christmas" all year round.