Preperations started pretty much from day 1 of Modi's govt, this move is the culmination of a lot of efforts and seperate but linked strategies over the past 2.5 years; SIT on black money, numerous black money disclosure agreements with key nations (US, Switzerland, Mauritius etc), bank account roll out to 300+ million Indians who had not been part of the digital banking sector, voluntary depositiing of black money last month (said to have raised $10-15BN for the GoI).
God damn, I have been a critic of Modi at times but this is devestatingly cunning, hatsoff to the PM. Not many nations in the world can boast of having a leader as visionary as this that puts their nation's interests first.