Royal Mail says you should post a second class letter or parcel by tomorrow to ensure it arrives by Christmas.
There is more time for first class post, with a recommended deadline of next Tuesday, 20 December.
The special delivery suggested cut-off is Thursday, 22 December.
Of course, post needs to be collected on the respective day so check what time the last collection is from your local post box.
Royal Mail stresses these are only the "recommended" last posting dates for UK deliveries, as opposed to a definite cut-off.
It is worth sending post as quickly as possible, especially if the cold snap delays deliveries.