I found a circular wheel with two dials that was originally meant to help people work out their mortgage repayments, which means I didn't have to start from scratch (thank you National Bank!).
I (re)used a piece of craft paper, and traced around the outside of the wheel. Then comes the hard part. The outside wheel has 28 increments with the 1 to 28 days in a month (days 29, 30 and 31 are shared with days 1, 2 and 3). So I worked out the spacing that I needed between each day with this formula: c=2πR, where c= circumference and 2R= diameter. When you divide c by 28, you get the width for each day.
It looks great on our black board!