Although the word Novembery
isn't really a word (you won't find it in the dictionary, anyway), an author would use it as an adjective describing something that feels like the month of November. How can something feel like a month? Just think of what you typically see in the month of November — falling temperatures and cool breezes, leaves falling off the trees and littering the ground, family gatherings for Thanksgiving, and so on. An author might say that there's a "Novembery chill in the air," which, because you know what the air in November feels like, enables you to imagine what that chill might feel like (and it would be quite different from a "Decembery chill in the air," which would be a lot colder).
Authors often use their literary licenses (or poetic licenses) to make up words like this — based on other real words — to better convey what they want to say. These "licenses" refer to the freedom that writers have to express themselves artistically — that is, in ways that deviate from the normal rules of writing style and form.