Is Visiting really that Important for Admissions?

<p>Does anyone know the latest one can visit a campus in order to have it reflected in one's admission decision for a January 1st application deadline?</p>

<p>I haven't visited any schools yet, and have no idea how schools track visitations. Hypothetically, if i visit Northwestern University January 6th, and the adcom reviews my application January 7th, will they know i visited?</p>