We are in Southern California so anytime is a good time for us, but we do always avoid storms so we at least get a few days before any rains. In colder climate I would also be aware of temperatures getting to cold before the concrete has fully cured which takes up to 60 days. About 75% of curing happens in first seven days so its good to have a good weather window of a least a week. I would prefer to pour foundation in spring which also allows good weather for framing and other trades. Hope that helps.
Dry and above 55 F for 3 days