When is a car tax disk valid from?

sassy_one

You cannot use a tax disc until the start of the month to which it is valid for, in this case, the 1st of August.

mikey72

I always put the new one in when I buy it, but from the DVLA website, I shouldn't.
Displaying your tax disc

Your vehicle tax disc must be displayed on the passenger side (kerb side) of the vehicle’s windscreen. If there’s no windscreen or you have a motorcycle or sidecar, you should display the tax disc on the kerb side of the vehicle.
A current tax disc must be displayed on the vehicle it was bought for. If you buy a new tax disc before your current tax disc expires, you should not display the new one until the old one has expired. The new tax disc is only valid from the first day of the month you have taxed from. The maximum penalty for failure to display a current tax disc is £200. The tax disc can not be transferred between vehicles

http://www.direct.gov.uk/en/Motoring/OwningAVehicle/HowToTaxYourVehicle/DG_10021514