Did a little research to see when the last $1 per gallon sale was this time last year. According to slickdeals, the sale began on 6/26/2011 - the last Sunday of June. Sale ended 7/23/11.
http://slickdeals.net/f/3045489-Petco-1-Per-Gallon-Aquarium-Sale-6-26?
http://slickdeals.net/f/3045489-Petco-1-Per-Gallon-Aquarium-Sale-6-26?