Flag day (computing)

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.{{cite web| url = http://www.catb.org/jargon/html/F/flag-day.html | title = flag day| accessdate = 2008-09-22}}

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.{{cite book|author=Eric S. Raymond|title=The New Hacker's Dictionary|url=https://books.google.com/books?id=g80P_4v4QbIC&pg=PA192|year=1996|publisher=MIT Press|isbn=978-0-262-68092-9|pages=192–}}

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.Jon Postel, [https://www.ietf.org/rfc/rfc0801.txt NCP/TCP Transition Plan, RFC 801]