Aside from his full-time job at a hospital, my husband works at a doctors' office one day a week. The doctors are hosting a holiday dinner for the office employees & technicians this Saturday night at what is described as a "classic tablelcloth restaurant" and, as the website claims, the restaurant "accords its guests with an upscale ambience and impeccable service, while maintaining affordable prices." I asked my DH what the office requested for attire, and he said "festive".

Now, I assume that jeans and a light-up Rudolph/Santa sweater would not be appropriate...

But does that mean I should wear a black cocktail dress or something more business-like (skirt or slacks with a blazer)? My DH will probably wear slacks and a collared shirt with a black sweater over it.
I'm a stay-at-home mom, so I really don't know what I should wear as the spouse of an employee. I have dressy clothes, though, so I don't feel like I should go out and buy something just for this...however, I also don't want to show up over- or under-dressed!
Advice, anyone? Thanks!