As with many BBC Christmas specials, the episode is shaping up to be something of a stand-alone adventure with the events largely taking place in 1895 Victorian Era England. This move and the year in particular isn't a random choice by any means. As co-creator Mark Gattis recently confirmed the date, the BBC explained that 1895 was the hight of Sherlock Holmes' popularity at the time and it was the year that creator Author Conan Doyle killed the character, only to resurrect him later on - much the same way the showSherlock did during Season 2 and Season 3.
This significant date in the history of the Sherlock Holmes character has already been hinted at in the show. Acutely observant Sherlockians noticed that this was the number that was shown in Sherlock's second season when we got a look at the page hits John Watson's blog was getting. This Victorian Era adventure appears to be something that the show's creators may have had brewing for some time now.