Update and save vacation at the end of the month

I have an Excel sheet where I track the vacation taken. At the end of each month I want to keep trace of how many days each employee had used. Vacations taken are in C4 - C20 and end of month date are in D4 - O20, each column being a month end data. How do I copy the data from C4 - C20 to each column at the end of each month?