Here's a hypothetical situation for you.
You're on campus and in your office (except for teaching) all day long, eight or nine hours at a time, three and sometimes four days a week. In fact, when you stop by your colleagues' offices, no one is usually there.
However, by some invisible decree, meetings are held on the one day you're not there.
Someone mentions a committee that you've volunteered for but expresses concern about your "limited availability" and asks, "Which day [!] are you on campus?"
To use the title of that children's book, what do you say, dear?