Optimization Problem.

I'm trying to solve a nurse scheduling problem. How can I build a mathematical model/algorithm for it? The problem asks to find which nurse should work on which day (M,Tue,Wed,Th,Fri,Sat,Sun). There're 20 (n_1, n_2,...,n_20) nurses in total. We need 2 nurses from Mon to Fri and 4 nurses from Sat to Sun. It's required that every nurse cannot work more than 4 days in a row. We also need to consider sick leaves or vacations. For example, n_3 is on holiday leave from Wed to Sat, etc.. Thank you so much!!!