The fall semester (September through November) and spring semester (January through April) are when Kent is most alive. The restaurants and bars are full, the downtown has energy, and the campus is worth walking. If the character of a college town is what you're after, this is when to come.
Each May brings events around the May 4 anniversary — lectures, vigils, and public programs on the Kent State campus marking the 1970 shootings. The anniversary date (May 4) is the focal point; if you're particularly interested in this history, it's worth checking the university's schedule for that year.
Summer is quiet. Most students leave, some businesses reduce hours or close, and the town loses much of what makes it distinctive. It's still perfectly functional, and the Cuyahoga Valley is at its best in summer — but Kent itself is subdued.
Avoid football Saturdays during the fall if you care about parking. The stadium fills up and the traffic around campus becomes genuinely unpleasant.