[wiki] Flag day (software)
Flag day ... 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 it doesn't work—reversing the change is similarly difficult and expensive.
This usage originates from a change in the Multics operating system's definition of ASCII, which was scheduled for the U.S. holiday, Flag Day, on June 14, 1966.
Another flag day was January 1, 1983: when the ARPANET changed from NCP to the TCP/IP protocol suite.[