14 CFR § 259.8 - Notify passengers of known delays, cancellations, and diversions.
(a) Each covered carrier for its scheduled flights to, from or within the U.S. must promptly provide to passengers who are ticketed or hold reservations, and to the public, information about a change in the status of a flight within 30 minutes after the carrier becomes aware of such a change in the status of a flight. A change in the status of a flight means, at a minimum, a cancellation, diversion or delay of 30 minutes or more in the planned operation of a flight that occurs within seven calendar days of the scheduled date of the planned operation. The flight status information must at a minimum be provided in the boarding gate area for the flight at a U.S. airport, on the carrier's website, and via the carrier's telephone reservation system upon inquiry by any person.
(1) With respect to any U.S. air carrier or foreign air carrier that permits passengers and other interested persons to subscribe to flight status notification services, the carrier must deliver such notification to such subscribers, by whatever means the carrier offers that the subscriber chooses.
(b) For its scheduled flights to, from or within the U.S, within 30 minutes after the carrier becomes aware of a flight cancellation, a flight delay of 30 minutes or more, or a flight diversion, each covered carrier must update all flight status displays and other sources of flight information that are under the carrier's control at U.S. airports with information on that flight irregularity.
(c) If an airport-controlled display system at a U.S. airport accepts flight status updates from carriers, covered carriers must provide flight irregularity information to that airport for the carrier's scheduled flights to, from or within the U.S. within 30 minutes after the carrier becomes aware of such a change in the status of a flight. Flight irregularity refers to flight cancellations, flight delays of 30 minutes or more, and diversions.