Worst nightmare - unless Mrs. Patspost is a fan of shopping/dressing up (not all of us females are!)
As suggested, a cocktail dress is good and gown OTT. A summer dress would not cut quite cut it. Think elegant wedding attire. I'd generally be going for a dress rather than trouser suit - which I'd comfortably wear to a regular wedding.
Old faithful, LBD (little black dress) - if subtly (don't want to look like mutton dressed up as lamb now) blinged up, that is shiny clutch bag, appropriate jewlery, etc, works well.
Have a look at the back pages of the irish glossy magazines - I'm not actually suggesting buying them, but if you're at the Dr/Dentist!
p.s. trust me, it's not easier for us girls - specially those who aren't keen on dressing up/shopping. It would be far easier to be told, wear a black suit. But maybe that's just me.