May have read it wrong, but if you work 2 monthly and you say you will be busy Jan 10 to feb 20, then surely you have more than 3 days work in December?
Go canvassing, you may pick up one offs that only want a pre-Xmas clean, you may pick up regulars....have you contacted your existing custies to see if they want insides doing?
also sounds like you are planning around the weather...don't!... the weather will do what the weather will do! be aware of it but don't let it rule your work.