It was nothing to do with Christmas time first, but with Epiphany (i.e. Twelfth Night) and Easter, since the cake is a great mixture of the Twelfth Night Cake and Easter Cake.
Celebrating the last day of the Christmas season, 5 January or Twelfth Night, included the tradition of making a cake for the occasion. The ingredients were flour, eggs, dried fruits, nuts and the seasonings of winter.
When marzipan and ‘Royal icing’ were added, the Christmas Cake was born. Royal icing means the decoration of a sugary paste including lemon juice, first used by British royals to decorate their wedding cakes.
Let’s see how to make it the Victorian Way (via English Heritage)
Another traditional recipe can be found HERE
GREAT BRITANNIA 