For my Sweetfire Creations mailings, I like to know when is the last day I can mail something for it to arrive before Christmas. I found a really good article in the Washington Times about that. Here is the link: Know your holiday shipping deadlines for Christmas gift delivery by Dec. 25
I do mostly first class mail and that deadline is December 20th this year. Please note that it does take a little time to prepare the item for shipping and getting to the post office so try to add a cushion of at least a day to be on the safe side.