Technically, an airline can cancel a flight at any time for safety or operational reasons, but they cannot do so without legal consequences. Under regulations like EU261/2004 (and similar 2026 U.S. DOT rules), if an airline cancels a flight with less than 14 days' notice, they are generally required to provide you with a full refund or a seat on the next available flight. Furthermore, if the cancellation is within the airline's control (like staffing issues or technical faults) rather than "extraordinary circumstances" (like weather), they must pay you monetary compensation ranging from $250 to $700 depending on the distance. While the airline "can" cancel without warning due to an emergency, they are legally mandated to communicate the change as soon as possible and are liable for your "duty of care," including meals and hotel stays if you are stranded.