As Morrow says,
For most of the year Solar Cause lives in the warm, yellow sun. He watches over the far-away Earth, making sure that it spins all through the day and all through the night. Once a year, the Earth's clockwork starts to run down. Day and night get so out of balance that one almost falls into the other! On that crazy day, Solar Cause reaches across the vastness of space and draws the Earth a little closer to the sun. He climbs down to fix our shivering little planet--and brings with him a bag full of presents for good boys and girls.
Solar Cause comes to Earth on that special date to set things right. With a gentle nudge to the North pole, he puts day and night back onto course and sets the Earth whirling into another year. But each of our hearts spins on its own, private pole. Like the Earth they will, if left alone and untouched, tumble into weariness and confusion. When he comes to fix the Earth's orbit, then, Solar Cause nudges each our hearts to set them right, too.
Morrow doesn't detail how we ought to decorate our Ex-Mass trees. I think I've found just the thing, though: a colorful collection uniform polyhedra. Both beautiful and orderly, they exemplify the meaning of Ex-Mass. And for the top of the tree, the only uniform polyhedron that rises above Wythoff's taxonomy: the great dirhombicosidodecahedron! (Don't be fooled; its apparent Wythoff symbol--(|3/2 5/3 3 5/2)—is in fact only a pseudo one.)
How fitting, too, that the great dirhombicosidodecahedron resembles a blazing star! For, as Morrow explains, we call it "Ex-Mass" in celebration of the sun's conversion of mass into energy. Solar Cause alludes to that same transformation, when, in his traditional salutation, he references the byproduct of solar fusion: "He, he, he! Forward, upward, outward!"