That was not the question. Opposite of a sentence, not individual wordsBut your own answer is negating only the "is" individual word in the sentence.
And since "bright sunny" is a qualitative term rather than a binary state, "not a bright sunny day" is not it's opposite at all. The negation of a qualitative term gives you "everything else" not its opposite.
;-)
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1