Zoey read $15$ books, one at a time. The first book took her $1$ day to read, the second book took her $2$ days to read, the third book took her $3$ days to read, and so on, with each book taking her $1$ more day to read than the previous book. Zoey finished the first book on a Monday, and the second on a Wednesday. On what day the week did she finish her $15$th book?
$\textbf{(A)}\ \text{Sunday}\qquad\textbf{(B)}\ \text{Monday}\qquad\textbf{(C)}\ \text{Wednesday}\qquad\textbf{(D)}\ \text{Friday}\qquad\textbf{(E)}\ \text{Saturday}$
The process took $1+2+3+\ldots+13+14+15=120$ days, so the last day was $119$ days after the first day.
Since $119$ is divisible by $7$, both must have been the same day of the week, so the answer is $\boxed{\textbf{(B)}\ \text{Monday}}$.