In weather folklore and the Gaelic calendar, November sets the scene for winter. As a result, Christmas Day falls within the middle of the winter season which is a time when people would expect winter to be well in place. You'd open the curtains on Christmas morning and find deep snow cover in your region. However, this is all false meteorologically and astronomically. Meteorologically, winter starts on December 1st and ends on February 28th/29th. Astronomically, winter starts around December 21st and ends around March 20th. Therefore, Christmas Day is near the beginning of the winter. This is the least likely part of the winter to record snow but people expect it to be freezing cold and snowy then gradually get warmer after. This is just not the case in reality. Anyway, I just wanted to bust those myths you hear constantly every year.
In the UK and Ireland, a white Christmas is considered when one single snowflake falls somewhere in either country during the 24-hour period of December 25th. This is in stark contrast to what we usually associate a white Christmas with.
In the UK, a white Christmas typically occurs every 1.42 years on average including 38 times in the last 54 years up to 2018. However, keep in mind that there'd be a lot of variation across the country during these Christmasses so each place would have a different return period whilst the 1.42 years figure refers to the UK as a whole. Widespread lying snow on the ground at 9 a.m. on Christmas Day is far rarer and exceptionally so. This kind of White Christmas (which is the definition people refer to a White Christmas as) has happened only 4 times in the last 51 years up to 2018 which gives an average of every 12.75 years. The graphic below by the UK Met Office shows a history of Christmas Day including the record White Christmasses.
Republic of Ireland
In Ireland, a White Christmas typically occurs every 3.56 years on average including 16 times in the last 57 years (1961, 1962, 1964, 1966, 1970, 1980, 1984, 1990, 1993, 1995, 1998, 1999, 2000, 2001, 2004, 2009 and 2010) up to 2018 at least one of the synoptic stations. Like the UK, there is lots of variation from year to year and not every place has the same return period of a white Christmas. 9 of these years (1964, 1970, 1980, 1993, 1995, 1999, 2004, 2009 and 2010) had snow lying on the ground at 9 a.m. on Christmas Day. The maximum snow depth recorded on Christmas Day was 27cm at Casement Aerodrome (Co. Dublin) on Christmas Day 2010.
Dublin Airport had 12 White Christmasses out of 77 years (1950, 1956, 1962, 1964, 1970, 1984, 1990, 1993, 1995, 1999, 2000 and 2004) up to 2018 which gives an average of every 6.42 years. Notice the 14 year gap from 1970 to 1984 without a white Christmas. As a comparison, it was every 5.90 years up to 2011 showing how absent a white Christmas has been at the station since its last one in 2004. 2010 was not considered a white Christmas as no snow fell at Dublin Airport but the station had a snowfall depth of 20cm on the day and this was the only Christmas Day with snow lying on the ground at 9 a.m. at the station since 1941.