Mistake

Today is the special day, because my friend and I do somthing special thing.
My friend and I were waiting other friends, and we still couldn't see any one about fifteen minutes later. My friend asked me, '' are you sure it is 11 o'clock ?" "Yes, I'm very sure." I said. "Well... are you sure today is 13th?" She said. "Uh....." I slienced and I took my cell phone from my bag to check the date. "Ha...today is 12th..." All of sudden, both of us all laughed. What about the end of our meeting? It was called off just now, because one of us catched a cold.