Definition of flag days:
(n) :
(British, Ireland) A day on which a registered charity raises money, usually by selling small lapel flags.
(n) :
(Hong Kong) A day on which a registered charity raises money, usually by selling small stickers.
(n) :
(software) A day designated for changing to an incompatible system.
(n) :
(software) A change which is neither forward nor backward compatible.
(n) :
A day commemorating the adoption of a national flag.
(n) :
(US) June 14, for the adoption of the US flag in 1777.