1. Gather the appropriate number of whole walnuts for your "calendar" — whether that is 12 for Christmas, or eight for the nights of Hanukkah. You may want to have a few extra walnuts on reserve in case a few of the shells crack.
2. Using a dull table knife, carefully pry open each walnut so you have two intact halves. The trick to doing this without cracking the shell is to insert the knife into the seam at the back where there is often a tiny opening. Be very careful not to slip and cut your hand! Keep each walnut half paired with its mate, otherwise you'll have a pile of ill-fitting walnut shells.