Experts and BHG readers answer.
Thanks for writing. It's best to spread winter mulch on your garden AFTER the soil freezes (when daytime temperatures no longer get above 32F/0C).
The point of winter mulch is to keep the ground cold and frozen all year; most of the damage to plants comes on warm days when the soil starts to thaw, then freezes again at night. Mulch keeps it from thawing so your plants stay protected.
----Justin, Senior Garden Editor, BHG.com