Would my work place let me take a 3 week vacation during December/Christmas?

Well if this is a busy time of year for Macy's then the likely answer is no if they have no one else to fill your spot? Usually after the holidays thing start to really slow down could you wait until then? Three weeks is a long time out of work unless you have been injured or something. I would not stir up things and call HR right now, You could always quit "before" you go on vacation.