Airlines typically launch sales late Monday or very early the next day, and we start seeing them Tuesday morning. Other airlines – ones that compete on the same routes as the sale-launching carriers – then swing into action to match their competitors’ fares. It’s not because they’re nice guys, it’s because they want your business too and don’t want to end up on page five of a shopper’s airfare search. This Tuesday “matching activity” is essentially complete by 3 p.m. eastern.
Is this the Only Day to Find a Deal? No. Shopping on Tuesday is the best and most consistent way but airlines can and do come out of left field with good prices from time to time.
To take advantage of great fares that can appear at any time sign up for airfare alerts.
Based on farecompare article.