I made 10 pockets from odd scraps of thin card, and used the inside covers so that I had 1 place for each of the 12 months.
Used a circle punch to cut a lip from sides to make it easier to get each month out.
I used a spreadsheet on my computer to make a line for each months with dates.
Printed it into long strips then I could fold then up to slot into each page in book.
Try It Tuesday - Letters and Numbers