Sorry for this generic answer but it is the truth.
When the snow gets deep enough to cover their feed up high to where they can''t get to it. It might start in October or December, it just depends on the year. I know of a herd in Utah that winters at 9500' + just because the wind blows the snow off of the high ridge that they live on and there is feed all winter long.
Like this last winter for example. Where I live here the winter was so mild I never saw any elk come down as usual. In general there are several hundred not far from the house by December/January but they never made it all this year.
There it sits. Alone and forgotten in a desk drawer or maybe in the bottom of your hunting pack. The lowly compass. Primarily initially replaced by the hand held GPS and now even by the new “smart” cell phones that include GPS, electronic compass – even real time imaging on aerial maps! Once the friend of every hunter and now the companion of few. It is not glamorous or glitzy, that is for sure… no bells and no whistles. Aaaaah, but let’s not rush to forget our...