"Завтра є понеділок." should have been accepted! "Є" = "IS" !!!
English copulas (is, are, ...) usually do not translate to Ukrainian. "Завтра є понеділок" sounds unnatural for standard Ukrainian, but it can be used in some dialects.
Also, I would have said this properly as: "Завтра буде (will be) понеділок." Which is correct also (future tense).
Yes, it is grammarly correct, but a bit longer.
Also i think you can drop буде altogether. "Завтра - понеділок". At least in conventional Ukrainian