When laying natural stone paving in the winter it is important to take precautions against the effects of frost. The worst case scenario is that the mortar on which the paving is laid fails to go off. To prevent this happening it is necessary to use a mortar frost proof. This will accelerate the speed in which the mortar goes off. It also helps to cover the mortar over with a sheet in the evenings to prevent the frost from getting into the mortar.
Particular care needs to be taken when installing less porous materials such as slate paving and granite paving. It may be necessary to use a bond bridge in these situations to aid adhesion between paving material and bedding layer. This should not be necessary with sandstone & limestone paving as these materials are generally more porous.
The rule of thumb is that in the winter everything is slower and mortar in particular takes longer to go off.