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 it doesn't work—reversing the change is similarly difficult and expensive.

This usage originates from a change in the Multics

Multics

Multics was an influential early time-sharing operating system. The project was started in 1964 in Cambridge, Massachusetts...

 operating system's definition of ASCII

ASCII

The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...

, which was scheduled for the U.S. holiday, Flag Day, on June 14, 1966.

Another flag day was January 1, 1983: when the ARPANET

ARPANET

The Advanced Research Projects Agency Network , was the world's first operational packet switching network and the core network of a set that came to compose the global Internet...

 changed from NCP

Network Control Program

The Network Control Program provided the middle layers of the protocol stack running on host computers of the ARPANET, the predecessor to the modern Internet...

 to the TCP/IP protocol suite.