Is it ok to go off for one lunch?

I'm on day 2 and plan to stay on for 10 days. However, on Thursday I MUST attend a luncheon - believe me if I could get out of it I would - and plan to have a bowl of lentil soup and some water. Then I'll go right back on the MC. Has anyone done this -go off for just a bit and then return? I wonder if it will have much of an effect, like gaining a pound or osmething. Day 2 but I have not lost anything yet.