Many countries set aside a day for patriotic activities to honor their national flag. In Argentina, this is June 20; in Australia, September 3; in Canada, February 15; in the United States, June 14; etc.
World Vexillology Day is different. It’s not about patriotism and it’s not centered on any particular flag. Rather, it’s a celebration of all flags and an invitation to learn about or otherwise show appreciation for them.