Q. What does flag day mean?
A. flag day – a software change that is neither forward-compatible or backward-compatible. It’s also costly to make and costly to reverse.
Usage: “If we add this new feature, will the users have a flag day?”
Flag day came into usage when a major change was made in 1966 to the Multics timesharing system to convert from the old ASCII code to the new one. Since the update was scheduled to take place on June 14, which is the U.S. holiday Flag Day, the name stuck.
Source: New Hacker’s Dictionary