The United States Postal Service gives tomorrow (Thursday) as the last day for sending First Class mail for Christmas delivery. That's means items should be ordered today at the latest or upgraded to Priority Mail.
It has been my experience this year that even the Priority Mail has been running quite late so get your final orders in with all on-line vendors just as soon as you can.
International ships dates have already passed.
That probably means I should mail those Christmas letters . . .