A flag day is a flag-related holiday—either a day designated for flying a certain flag (such as a national flag), or a day set aside to celebrate a historical event such as a nation's adoption of its flag.

Flag days are usually codified in national statutes, however, in some countries a decree by the head of state can also order a flag day. The statute or decree may specify locations where flags are flown and how are they flown (for example, at full- or half-staff); alternatively, custom may prevail.^[1]