I'm waiting till the temps are consistently above +8dC, since my normal tires are summer tires and become hard/less grippy. Likely mid April for me.
If your regular Caravan tires are all-seasons, which I presume they are, you're probably fine changing anytime. They're less effective in the snow and ice, but will do the job. You may want to keep them on till April just to alternate wear between your summers and winters and extend the life of both, but they're likely priced similarly so doesn't really matter.