I work as a contractor. The expectation is I work Monday - Friday 8am - 5pm and to inform them if I'm not going to be in on a given day. This is to be on the same schedule as the rest of the team. They have verbally told me on multiple occasions it's ok if I take days off when I feel like it. Should I get this written into my contract? What language should be used? Or is there no point, since if they're unhappy with my work they could terminate my contract for any reason? I don't get any holidays off.
In the USA, if you're a freelance contractor, your clients have limited ability to control your schedule, place of work, and other aspects of your arrangement with them. Misclassifying you could be a potential issue for them with the IRS.
That being said, you don't need anything in your contract that stipulates your time off. That's completely at your discretion. What I try to do is give my clients ample notice if I'm going to take an extended leave, like for vacation. If I'm simply not feeling well or have other things going on and will be away for a day or two I generally try to give them as much notice as possible. In the case of being ill, that's usually the day of.