Okay thank you FarmerJ and Alaska Landlord for the very prompt responses to my question - I really appreciate it!
To clarify, I would consider the apartment uninhabitable for the day because there will be no running water or drainage available at all, the repair work is to exterior plumbing lines. This is a non-issue for the other tenants on the same property because they are away at work all day. However, I and one other person are stay-at-home people and it's a fairly major inconvenience to spend the entire day hanging out in coffeeshops, the library, etc. I'm working on finding a solution on my own (a friend may be out of town), but just in case, I'd like to know what my landlord's legal responsibility is because he's a decent person and we've have a good relationship, but he's just an inexperienced, amateur landlord who inherited the property. Once something is pointed out to him as his legal responsibility, he'll suck it up and do it, but it's usually up to me to explain it to him!
Any further advice or info would be most welcome!