A flag day, as used in system administration, is a change which requires a complete restart or conversion of a sizable body of software or data. The change is large and expensive, and—in the event of failure—similarly difficult and expensive to reverse.^[1]

The situation may arise if there are limitations on backward compatibility and forward compatibility among system components, which then requires that updates be performed almost simultaneously (during a "flag day cutover") for the system to function after the upgrade. This contrasts with the method of gradually phased-in upgrades, which avoids the disruption of service caused by en masse upgrades.

This systems terminology originates from a major change in the Multics operating system's definition of ASCII, which was scheduled for the United States holiday, Flag Day, on June 14, 1966.^[1][2]

Another historical flag day was January 1, 1983, when the ARPANET changed from NCP to the TCP/IP protocol suite. This major change required all ARPANET nodes and interfaces to be shut down and restarted across the entire network.^[3]