Answer:
Santa shakes his belly like a bowl full of me
Blade to blade I always win My foe laid low and even. Yard by yard through thick and thin I'm busiest in season.
Lovely and round, I shine with pale light. Grown in the darkness, a lady's delight. What am I?
I only exist when you are here. Where you never were I can never be.
I have roots nobody sees. I am taller than trees. Up, up I go but I never grow. What am I?
No matter how little or how much you use me, you change me every month.