Can you? Sure. Should you? Probably not.
Doing one day of volunteering is unlikely to be impressive enough to bother devoting space to. It's not enough time to really demonstrate or develop skills and it's not enough time to demonstrate commitment to a particular cause. It's lovely when people want to volunteer but if you're going to put it on a resume, it should really be a much more substantial commitment. If you list a single day activity, you're likely telling whoever is reading your resume that your experience and skills are so minimal that a few hours of work is one of your biggest selling points.
Realistically, you likely want to highlight whatever it is that lead to your professor asking to volunteer in the first place. I'm not sure how to get from "exceptional academic achievement" to being asked to repair some computers unless you happen to be in a computer repair program. You'd want to emphasize that academic achievement and whatever lead to your professor believing you'd be good at repairing computers not the day you spent volunteering.