Definition from Wiktionary, the free dictionary
Proper noun 
- A day commemorating the adoption of a national flag.
- (Australia) September 3, for the first flying of the Australian flag in 1901, Australian National Flag Day.
- (Canada) February 15, for the inauguration of the Canadian flag in 1965, National Flag of Canada Day.
- (US) June 14, for the adoption of the US flag in 1777.