Open a vacation savings account. Schedule monthly transfers from your primary savings to the vacation account. Problem solved. You'll have money set aside when it's time to buy the APs, and will earn a few dollars of interest along the way.
There is no set pattern. If memory serves, the original AP discount went into effect around April or May 2005. The amount of discounts have been adjusted whenever ticket prices increased. The special $399 PAP was announced in September 2012 and was valid Oct-Dec.