From the government web site:
When and where to display the disc
A vehicle tax disc shows the registration number and the date up to which you have paid vehicle tax. It comes into force on the day it is issued. But, if you apply for a tax disc in advance, it comes into force on the first day of the following month and you should not display it on your vehicle until then. You must display this disc on the left-hand side of the vehicle's windscreen. If the vehicle does not have a windscreen fitted, you will need to display the disc on the passenger side. For motorcycles, the tax disc should be displayed on the near-side of the bike. If a side car is fitted to the motorcycle, then the tax disc must be displayed on the near-side of the handlebars or on the near-side of the side car.