December 20 for standard small First Class mail packages
December 21 for Priority Mail packages
December 15 for Parcel Post (really only applies to my cigar box guitars)
December 10 for shipments to Canada.
Now keep in mind that I can't 100% guarantee that the USPS won't lose something or misplace it. Last year around Christmas there were some shipping anamolies... but these are the dates they give so they are the ones I am putting forward.