jmc

Let the date behind $C$ be $x$. Now the date behind $A$ is $x+1$, and after looking at the calendar, the date behind $B$ is $x+13$. Now we have $x+1+x+13=x+y$ for some date $y$, and we desire for $y$ to be $x+14$. Now we find that $y$ is the date behind $P$, so the answer is $\boxed{\text{P}}$