flag day

Short definition from Wiktionary

Noun

1. 1
   A day on which a registered charity raises money, usually by selling small lapel flags.
2. 2
   A day on which a registered charity raises money, usually by selling small stickers.
3. 3
   A day designated for changing to an incompatible system.
4. 4
   A change which is neither forward nor backward compatible.

Proper noun

1. 1
   A day commemorating the adoption of a national flag: September 3, for the first flying of the Australian flag in 1901, Australian National Flag Day.
2. 2
   A day commemorating the adoption of a national flag: February 15, for the inauguration of the Canadian flag in 1965, National Flag of Canada Day.
3. 3
   A day commemorating the adoption of a national flag: June 14, for the adoption of the US flag in 1777.