By me they've been doing it during the Q3 months of the year (July through September). When they first did it by me, they did it in August. The following year it was both August and September.
This past year though they changed the rules, only 10g, 20g, 29g, and (i think) 55g tanks were on sale though they were never available by me. 40g breeders, 20g-longs, etc, were no longer on sale.