OK. It has MOT. the insurance will start on the 20th. I will buy tax on the 20th. I just hope that i don't set off the tax cameras. The car will be legal, just wont have a disk in the window.
If you can and do buy tax online on the 20th, you will be ok to drive without the disc, there is a grace period. I usually print off the confirmation and carry it in the car until the tax disc just in case, but they say that's not necessary..
Berkshirelad. the car is not in the EU. I am going to get it legal before I drive it there. Thanks all for the answers. I am going to hope that there is a grace period for non-displayal.
As I said earlier, if you have only recently taken out the insurance to start on 20th Dec it may not yet be on the DVLA database so you won't be able to tax the car online. As others have said your only other option is to take the insurance certificate and MOT and any other documents to a Post Office on the 20th Dec.