It’s easy as pie: use some large stamps (I used “Simply Sweet” foam stamps by Doodlebug Design) and black ink (Staz-On) and stamp the first three letters of the month on a card (I chose colors which resembled the mood/temperature of the months).

If you don't have large stamps, you can use a sharpie ;)

Now attach a printed list of the days to the card. Use good glue, otherwise they might curl upwards after some month like mine did…
Anyway, if you’d like to make such a calendar, I attached a free download for you as a PDF :)
Print it (12 copies of page 1 or, to be correct, 7 copies of page one, 4 copies of page two, and 1 copy of page three) and cut it to the width of your paint swatches. Trim to get an equal height. Choose a nice place for your calendar and attach it to your wall or door.
I always mark the current date with a little arrow (“Book Dart”), so I can see upcoming birthdays at a glance.

By the way, the icing on the cake is the cat on top of my door. I cut it from vintage wallpaper scraps. And it covers a dent in the wall *g*.

I hope you like this little tutorial!