The US Postal Service has a tool to tell you when you need to mail things to ensure delivery by December 24. It depends on what part of the world you’re in and the class of postage you use. We use first class, priority, and priority express mail (your choice, subject to weight of the order). Please scroll down and read the weasel words though. These cutoffs are what should work in most cases, but the USPS is not guaranteeing anything. Last Christmas it took a priority mail package 10 days to reach us, when it should (supposedly) never be more than 3 days.