Maybe "a" is only used for some other time? Maybe it's "We eat sausage at November.", meaning something like "We eat sausage when we arrive at November." and "We are eating...." would use "...in novembre." instead? I'm guessing.
Otherwise, it's probably just that the more common meaning is "We (always/usually) eat sausage in November.", in which case "We are eating sausage in November." should probably be accepted too.
Can anyone confirm if one of these guesses is correct?