Devil's Advocate
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The first time I saw a monthly fare calendar was on Southwest.com sometime in the early 2000's. Not every airline provides their own monthly calendar but most of them publish real time fares to a global booking service which can be read by third parties. Google's travel service is based on the ITA engine and shows several months of fares from hundreds of airlines with lots of user customization that puts Amtrak's clumsy low effort booking process to shame.The airlines show fares for each day for a month? When did that start?
https://www.google.com/flights
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