BBC cleverly makes a 7 minute mini episode for its fans, which i think is a great gesture. It says "He is coming!!!". Yes, Sherlock is coming back, and Watson is going to be in for a shock as new games begin. Detective Lestrade is confused by his assistants interest in the different investigative cases across the world. He is fixated on Sherlock not being dead, while Watson receives an old Birthday message from Sherlock.
The episode is called 'Many Happy Returns', and lets wait to watch how happy our viewer are going to be, when 'Sherlock' comes back on the 1st.
See the mini episode here :