Simply come back each day of a month, starting on the first, and complete at least one puzzle each day. You will need to do this each day of the month.
If you're starting this after the first of the month, then you can set your date back to the first and complete one puzzle. After that you need to close the game and then set the date for the second of the month. Open the game back up and complete another puzzle for that day. Repeat this until you get to the day that you are on and then finish it normally for the rest of the month.
This method can be used on any day of the month, so if you start the game on the last day of the month, you could unlock it the same day.