I've never really seen a 'big sale' in cycling accessories. Sometimes there will be a rack of heavily discounted stuff but it's never things I'm really accessories. Panniers, lights, and other utilitarian things never really get on sale, as far as I know; you may be a little more lucky with clothing but the cycling clothes I buy also are those that come back year after year, so no deep discounts there.

I hope others have more helpful answers!