Dear Diana: What Exactly is Black Tie Optional?

Consider it an excuse to dress up.

I’m heading to a friend’s wedding in Portugal this October, right after fashion month. By the time I land in Lisbon, I’ll have three weeks plus worth of laundry jammed into two suitcases along with whatever panic purchases I’ll have bought along the way. (DSM London and Paris watch out!) Also in my luggage? A black tie-ish look for the wedding, as the d…