Help with Calendar

I have this calendar that I made with the help of a lynda.com tutorial. I understand the code, but... I 'm doing something a little different. I would like the checkboxes to appear in each month's calendar. I'm not for sure why this isn't working, sot hat's where you come in. It's works well for the current month, but when you click the previous and next buttons, they disappear. please help.