To work properly in cold weather petrol has to evaporate more readily than in the summer and in the summer the petrol needs to be less volatile.
Less volatile Summer grade petrol is produced by the refineries and by law must be sold by service stations in the UK between the 1st of June and the 31st of August.
Diesel has different seasonal problems and has to be changed in the winter months by law between 22nd of October and the 15th of March. During this time a cold flow additive is used to stop the waxy compounds in the diesel solidifying due to the low temperatures. These waxy compounds are needed to ensure good engine performance.
Hope this helps