From Margaret - Christmas Pudding and Stir Up Sunday.

Conservative though they were, the Victorians believed Christmas should be celebrated (although excessive drinking and frolicking were frowned upon). It was they who established the tradition of making the Christmas pudding on Stir Up Sunday, the fifth Sunday before Christmas.

A THOROUGHLY VICTORIAN INVENTION?
Following the marriage of Queen Victoria and Prince Albert, the Royal family celebrated Christmas with gusto and the rest of the nation followed their example. Charles Dickens has certainly helped plant Christmas in our minds as a very Victorian custom.

“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 a half a quartern of ignited brandy, and bedight with Christmas Holly stuck into the top. Oh, what a wonderful pudding!” A Christmas Carol by Charles Dickens.

However, the Christmas pudding itself has much earlier origins.
The pudding we know today began life as a pottage. This was a kind of broth, including raisins and other dried fruit, spices and wine. It was thickened with breadcrumbs or ground almonds. Not dissimilar to the mince pies of yesteryear, it often included meat or at least meat stock.
The original ‘figgy pudding’ was almost unrecognisable from modern Christmas pudding
This plum pottage would be served at the start of the meal rather than at the end of the meal as we do today.
It was not until the end of the 17th century that the pottage took on a more solid appearance. It was served like a porridge or cooked inside a skin, like a sausage. Even then, it was more likely to have been sliced and cooked under a roasting joint and served alongside the main meal or as a starter – not a dessert.
During the 18th century, plum porridge would become associated with Christmas. It would be the Victorians who raised its prominence at the festive table.
Trinkets were stirred into the Christmas pudding mixture on Stir Up Sunday.
Stir Up Sunday was a family affair. Each family member was supposed to stir the mixture from east to west to honour the journey of the Magi. This ritual was also thought to bring the family luck in the coming year.
Originally the puddings would have been shaped into a sphere and boiled in a cloth. This practice eventually gave way to steaming the dessert in a pudding basin or elaborate mould, particularly in wealthier households. The traditional accompaniment to the Christmas pudding was a sweet custard or a hard sauce (nowadays known as brandy butter).
It was customary to hide a number of small trinkets in the mixture, a bit like the twelfth night cake.
From a-history-of-the-christmas-pudding