I ride all year 'round in New York City. You might miss a few days due to ice and snow, but usually the roads and bike paths are cleared pretty quickly.

Temps will be in the 20s and 30s, but you can dress for those conditions easily enough. Generally, the temps don't fall below 20 degrees for too long.

If you're commuting, you'll want a headlight and taillight of some kind. The city is pretty well lit, but you'll want to make sure cars and peds can see you. And maybe consider fatter tires and fenders as well.

You can try nycc.org for more tips.