。
She came to visit on Monday, not Sunday.
Literal
She [topic-は] Sunday [is-not-ではなく] Monday [on-に] [came-to-visit-訪ねてきた].
ではなく is the te-form of the negative copula ではない (with attributive な adjusting): 'not X, but ~.' Used between two noun candidates to negate the first and assert the second. 訪ねてきた combines 訪ねる ('visit') with directional ~てくる ('came (here) to visit').