It's the time of the year when it gets busy everywhere, not least at the post office. Royal Mail has advised following cut off days for Christmas posting services. Please note that due to high Christmas mail volume Royal Mail services may suffer from delays even without any snow stroms... Hence the earlier the better.
Please also note that orders should be placed at least a day before these mailing cut off days in order to allow time for order processing.
UK Posting days:
Tue 20th Dec - First Class Signed For
Sat 17th Dec - Second Class Signed For
Wed 21st Dec - Special Delivery
Wed 14th Dec - Standard Parcels
International Posting days:
Mon 12th Dec - Western Europe
Fri 9th Dec - Eastern Europe, USA, Canada
Mon 5th Dec - South & Central America, Caribbean, Africa, Middle East, Asia, Far East (including Japan), Australia and New Zealand
More info can be found here