Rachel, there's a famous saying that almost everyone knows: "If you can't beat 'em, join 'em". Why not extend your holiday spirit and invite your neighbors below to partake in the festivities, so instead of them being kept up by the sounds of festivities, they'll actually be part of the good times themselves? They can't complain if they're there having a good time with you.
But something tells us that if your neighbors below have already warned your building manager about contacting the authorities, there's a good chance you've been perhaps too loud for their liking on many past occasions (you yourself have called your friends "late 20s early 30s party animals", which doesn't bode well). End the new year with a good will gesture, perhaps with a small gift in hand (a bottle of wine?) and an outright apology for any inconvenience you might have imposed in the past and explain your plans. Usually this sort of considerate gesture takes people off guard, and more often than not, when you explain you're having a once a year party that they're more than welcome to attend, they'll be more apt to cut you some slack whether they attend or not. Because the truth is, your neighbors have the right to peace and quiet as much as you have the right to have a good time.
["Ugly Christmas Sweater Party" photo by paperladyinvites]