Dinner Pranskters

During dinner, Gavin put a grape in his fist to hide it from view. Here is the conversation that followed:

G- Guys, do you think that this grape has a stem or no stem?
E- No stem.
L- Stem.

Gavin opens his hand to reveal that the grape does, in fact have a stem.

E- I was right. I said stem.
L- No, I'm right because it's opposite day.
E- Oh.

I know nothing about this opposite day Lilah speaks of, but, Emma graciously accepted her defeat in the guessing game.