One way to celebrate Grav-mass is to decorate a tree with apples and other fruits. Glue them or attach them, but not too well! The idea is that occasionally a fruit should fall. Put them on the tree no more than 2 feet up, so that they won't get damaged or hurt anybody when they fall. Investigating and perfecting the methods for doing this is a great way expose a child to the process of scientifically studying the behavior of the physical world.
Another way to celebrate Grav-mass, in a group, is by telling each other about scientific knowledge and understanding, or discussing what kind of experiment could answer an unresolved question about the world.
We have one Grav-mass carol, We Three Laws. If you know of any others — or of any songs about science — please tell me about them: rms at gnu dot org.