I would advise fitting winter tyres around now, but bear in mind that in 4 months time, you'll need to buy a set of summer tyres as the winters will feel horrible when it warms up.
With the mileage you do, I reckon you could get 3-4 seasons out of a set of winter tyres, so it would be worthwhile fitting them to a second set of wheels and finding somewhere to store the other set. Some tyre dealers offer storage facilities providing you've bought your tyres from them, so it might be worth asking round.