Mid-December.
When I was a child it was always the Saturday before Christmas Day -unless that was actually Christmas Eve, in which case the previous Saturday. We were possibly on the later side of normal. Down on the 6th of January, in line with most of our neighbours.
I tried to stick to that but it was starting to seem grumpy and joyless, so we landed on the weekend of or after S1's birthday (the 12th). Still take them down on 6th January but invariably spend three or four days feeling like total outliers.
We do have an advent calendar and advent ring.