Just the cardboard tartan ones, Friday. In Scotland we usually wear tied garters, but there are elastic and velcro ones out there, too, and they're acceptable. Toss the tartan ones in the nearest bin.

RWM, think of what you will be wearing as a business suit. No silver buttons and no sporran with silver -- no bling. For your wife she should think elegantly casual. Preferably a dress or skirt and blouse, perhaps with a jacket. Played down accessories, too. No carryall for a handbag, no tartan sash and big broochie. You haven't said when this will be, but if there is a risk of coolness for her, the jacket may serve or even a woolen wrap.