I thought I saw somewhere in one of my clients' NetSuite accounts that there is a place to define business days vs weekends and holidays, but I can't seem to find such a thing in my current client's NetSuite account. Does anyone know of a feature like this in either native NetSuite or one of the commonly-used bundles?

In native NS go to Setup > Accounting > Shipping. Under the preferences tab at the bottom you can enter holidays. Perhaps this is what you're thinking of?

Hm, maybe, thanks. I don't really want to change it for shipping though, just for automatic billing.