Each community may have its own customs regarding sales. In Houston, many of the larger bike shops run sales in May. They are targeting the graduation present market, the summer rider market, the off to college soon market. Just a few models, at 10% to 20% off the normal selling price. But, a good selection of sizes of the sale models.
The last week of August and the first week of September is the "big" sale. Often 20% to 30% off. But, the most popular sizes are often sold out before the sale. What is left are the bikes that are too small, too big, or that are in a really awful color.
If I were buying a $500 bike, I'd rather buy it in May, and get exactly my size, and get my favorite color. Ride it and enjoy it during May, June, July, and August. Well worth paying an extra $100 compared to waiting to September, getting the wrong size and a bad color. (Black, or red, or silver are the "good" colors, of course.)