for you to finally swap over from a warm weather/summer wheel and tire setup to your winter wheel and tire setup?
me personally: once the first frost on the windshield of the vehicles hits (happened 5-6 days ago here in northern KY) its time to switch.
Ive got the fiancé's Clubman S and the ZHP to do. Her's is pretty quick and easy while the ZHP has the added step of adding spacers before putting the 135s back on.
I usually wash all the wheels and then throw them in dry storage.