Flag day commemorates the date in 1777 when the Continental Congress approved the design for its first national flag. From 1776 when seamstress Betsy Ross welcomed members of the Continental Congress (including George Washington) and then created a fabric of colors and symbols, our flag has been a symbol of freedom. At Election Integrity Network, the foundation of the freedom represented in our flag is the will of the people - expressed by safe and secure voting.
In 1885, a Wisconsin school teacher named Bernard Cigrand urged his student to write a birthday acknowledgement of the flag and published his own essay suggested an official Flag Day - starting a national trend.
In 1949, Congress and President Harry Truman made the day official.
More info from the National Flag Foundation: https://nationalflagfoundation.org/how-flag-day-became-a-national-holiday/