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.
Being a bowhunting I've always been fanatical about scent control. Whether it's sprays, carbon clothing etc. I've tried it. The best thing that I have found for eliminating human scent is Betadine. It's been used in hospitals and homes for decades as a defense against infections on minor scrapes and burns. It can typically be found near the pharmacy in any of the major "one stop shop" stores, and a bottle will last multiple hunting seasons.
Before every hunt possible I...