This is a prediction, and predictions are hard -- especially about the future.  Nobody knows, but I'll take a stab at it.

Day 0 is expected roughly around November 30th, plus or minus a week:

Day T-10: Four of the five ASIC manufacturers claim to be finalizing the design / optimizing / trying to resolve some nagging power issue / whatever, yet not a single person that pre-paid for an ASIC has one in-hand.
Day T-8: Difficulty of 3.5 million and exchange rate of $15 mean profitability is still acceptable for GPU miners paying average electric rates or less.
Day T-5: A lot more "N Ghash/s for sale" listings will start to appear [Edit: as-in total liquidation of N GHash/s of capacity]