Garters can be out when there's still snow! However, I would not be opposed to holding onto a wild snake until you get a 65-70 degree afternoon, if it's just a couple days. I definitely wouldn't put one out in the evening, with a freeze coming.
Something to consider if you're tempted to permanently keep a WC. There is a disease present in wild snake populations, a fungal disease, and it can spread to your collection and wipe it out. It's just not worth the risk.