This will depend on the species of tree you choose to have:
The best time to have a Nordman Fir delivered is towards the end of the 1st week of December - around the 6th. Nordmann Fir's last an average of 3 to 4 weeks indoors.
The Norway Spruce has a life expectancy of around a week indoors so we would not recommend this tree if you are looking to keep the tree up till the 6th, though if your house if cool it might just make it if you have it delivered after the 20th December.
Potted Frasier Fir's still have their roots so as long they are kept watered and away from heat sources they will happily look lush throughout the Christmas season.