What a bright day it is!
The subject is the dummy pronoun "it" and the predicate is "is what a bright a day". The distinguishing feature of exclamatives like this is that they have an exclamative phrase containing "what" or "how", which can be subject or non-subject:
(1) What unpleasant people work in this place. [subject]
(2) How strange a creature lives here! [subject]
(3) What awful weather we're having! [non-subject; object of "having"]
(4) How enjoyable the concert was! [non-subject; subjective predicative complement]
An exclamative subject occupies its basic position before the verb, as in (1) and (2), while an exclamative non-subject is obligatorily fronted, as in (3) and (4). Notice how the non-subject exclamative in (3) is object of "having" and predicative complement in (4).
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