The idea of plum pudding as a Christmas dish rose to prominence during the Victorian period, as seen in A Christmas Carol (published in 1843) shown in this illustration of the Ghost of Christmas Present from the first edition.
That Dickens saw the pudding as a unifying dish and a central symbol of Christmas cheer and plenty, is shown in his description of the Cratchit’s Christmas pudding:
In half a minute Mrs. Cratchit entered: flushed, but smiling proudly: with the pudding, like a speckled cannon-ball, so hard and firm, blazing in half of half-a-quartern of ignited brandy, and bedight with Christmas holly stuck into the top.
Oh, a wonderful pudding! Bob Cratchit said, and calmly too, that he regarded it as the greatest success achieved by Mrs. Cratchit since their marriage.
An 1850 London Illustrated news article also describes its prominence in similar terms:
The Plum pudding is a national symbol – It does not represent a class or caste, but the bulk of the English nation. There is not a man, woman or child raised above what the French would call proletaires that does not expect a taste of plum pudding of some sort or another on Christmas Day.