Improve the bracketed part of the sentence.
(It is) a pleasant day, we went for a walk.

This sentence uses a nominative absolute structure. Since the main clause is in the past tense ("we went for a walk") and the weather condition acts as a reason phrase, we use the participle construction "It being".
Using "It was" creates a comma splice (two independent clauses joined improperly by a comma).

"It being a pleasant day" correctly acts as an adverbial participle phrase.