To answer the questions that people are having about the arc itself, here are the answers.
Since the same questions about the gaiden have been appearing continuously, it has been deemed a good idea to create a thread that answers some of these frequent questions.
Please review this thread if you have any general questions about the gaiden before posting them in the discussion threads, as your question is probably answered here.
What is a gaiden?
A gaiden is a mini-arc in which explores a particular character or event within a series. This current gaiden explores the captaincy of Urahara and the origin of the vizards.
How do I recognize the chapters in the gaiden?
The gaiden began with Chapter -108, and the chapters in the gaiden have been denoted as -10X, with X being some digit less than or equal to 8. The chapter numbers are increasing incrementally one digit at a time, i.e., -108 was followed by -107, followed by -106, etc.
How long will the gaiden last?
In an interview, Kubo said that this gaiden should last for about 8 chapters (about two months). The gaiden will end in chapter -97.
EDIT: While not in these specific pages, the information about Bleach Gaiden was taken from part of the same interview with Tite Kubo that you see below. In the same interview it was also revealed that Unohana would have a battle soon. - GK
What time does the gaiden take place?
For chapters -108 through -105, the gaiden is set 110 years prior to the current action. In chapter -104, the gaiden advances nine years, 101 years prior to the current action.