An Post is advising the public to send cards and letters no later than today for guaranteed delivery by Christmas.
It says mail destined for the North of Ireland must be posted today.
Recent bad weather and backlogs could delay the arrival of mail recently purchased online.
Anna McHugh of An Post says it's best to put cards and mail in the post as soon as possible.
"If you still have to post letters to parts of Great Britain or the rest of Europe, you really should have done it by yesterday. So again, get those in the post today because there's all kinds of bad weather around the place. And there's huge volumes going through the system. So the sooner you do it, the better."
