What is a Christmas number one? The song that sold most singles that "Christmas"? Well sort of but not quite. In fact technically, it's the single that sold most in the week up to the Sunday before Christmas. To be even more precise it's sales between 00.01 Sunday and and midnight on Saturday. So that doesn't reflect what was sold the week(s) before or after. And that's all assuming that the single has been officially registered with the Official Charts Company, and comply with it's chart rules, for it to be eligible to be officially counted.
So timing is Everything
So the charts are always measured to a specific point in the week which is midnight on Saturday. However the day of the week on Christmas day falls will vary from year to year. This means that the chart could be finalised on Christmas day itself, or up to 6 days earlier. So it can be that with 6 days to go until Christmas then the Christmas No.1 is already decided. That variability has led to some near misses.
So for example, in 1990 Vanilla Ice was No.1 with Ice Ice Baby, but by the next Chart on 29th December, Cliff Richard's Saviour's Day was No.1. In 2010 it was Cannonball with Little Mix at Christmas No.1, with Military Wives with Gareth Malone had to wait a week to peak with Wherever You Are. In 2008 Leona Lewis's Run (20th Dec 2008) meant that Alexandra Burke' Hallelujah had to wait a week too (27th Dec).
Disparate Decades
Over the decades the nature of the Christmas No.1's have changed to become more disparate. In the 1950's seven of the ten were from either the Beatles or Elvis. For Elvis it was It's Now or Never [1960] and Return to Sender [1962], and for the Beatles with I Want to hold your hand [1963], I feel Fine [1964], and Day Tripper/We Can work it out [1965], Hello Goodbye [1967].
Once in a lifetime
The Christmas No.1 is also the opportunity for some of the quirky numbers to stand out. The test being that their airtime since had been miniminal. Benny Hill's Ernie- the Fastest Milkman in the West [1971], Donny Osmond's Long Haired Lover From Liverpool [1972], and then there was Mister Blobby in 1984. That includes those children's hits too....The JCB Song from Nizlopi in 2005, Can we fix it? from Bob the Builder [2000]. That said Band Aid managed to be the Christmas No.1 three times over 20 years with Do They Know it's Christmas in 1984, 1989, 2004.
Source and thanks: Official Charts Company
