can we take out az?
Yes. But it sounds better with the "az" to my ear.
Also, if you want, you can stick an "A" in front of "január".
Another alternative: "Idén a január nagyon hideg volt."
Duolingo is persistent in not accepting similar answers...
Pretty not equals to very?
Why do we need an article? We were taught that "idén" alone means "this year".