I've never heard of a specific date/time for the airlines to change rates. I know I've been booking a reservation before and the airfare changed 3 times while I was on the phone. I've been told that the airlines can basically do what they want, when they want. Rates are also likely to change based on the availability of a particular flight, and the demand of that time period.
I've also been told that if you purchase a ticket today at $500 and you continue to check rates and the fares go down to $300....you can go to them and ask them to reissue your ticket at the lower fare---you would need to pay the airline change fee (right now $100 for most carriers), but you would then get a refund of the remainder. Not sure if this is still the practice, and I've never tried this, but it sounds good.