I used round .5" ProMag magnets bought at Michael's 50 to a pack.
I wanted them to be strong magnets since for each of my squares I used two layers of card stock with the magnets in between.
Advent calendar (back) by XcessStamps - Cards and Paper Crafts at Splitcoaststampers
Advent calendar (front) by XcessStamps - Cards and Paper Crafts at Splitcoaststampers