Mix up

"When's your outside duty? I'd like to switch," my colleague approaches me in the hallway.
"Friday," I say and feel a smile spreading across my face. Friday is the last day of school and the only thing planned is a Leaving Ceremony. No duty. I dream of the last day. Few more to go.
"Lucky you," my colleague nods understandingly, "Whose on duty today?"
"I don't know. I know M's duty is on Mondays," I offer a snippet of unhelpful info.
"I'll ask her," my colleague starts walking off.
"You can't switch with her," I call after him. He stops and turns around looking puzzled.
How silly, I think.
"Her duty is on Monday. She was on duty already," I try to explain.
"I mean the second break not the first," he responds with an answer that makes no sense to me. We are all on duty during the second break.
"You can't switch with her because she was on duty yesterday," I feel that my speech slows a bit as if this makes information more clear.
My colleague looks completely lost hearing this.
Duh, if M was on duty yesterday and there are no Mondays left, how can you switch, I mentally shake my head when I see my colleague burrowing his eyebrows with confusion.
"But today is Monday. Only the fourth lesson. We haven't had the second break yet," he finally  says softly.
"It's Monday? Not Tuesday?" I realize I have been the one lost on space-time continuum. Feeling how I deflate, truth making me feel the pull of gravity, I probably resemble a statue of sadness. My colleague looks at me sympathetically, holding back laughter, steps closer, shakes his head, and offers a hug. I let him pat me on my back. I sigh.
Not Tuesday? Monday fourth period?
"You almost had me convinced," my colleague chuckles.
While my mind tries to restore order, laughter bubbles up and escapes my lips.
 
Whose ready for holidays?