In Britain, a flag day is a day on which people collect money for a charity from people in the street. People are given a small sticker to wear to show that they have given money.
Flag Day
uncountable noun
In the United States, Flag Day is the 14th of June, the anniversary of the day in 1777 when the Stars and Stripes became the official U.S. flag.
flag day in British English
noun
British
a day on which money is collected by a charity and small flags, emblems, or stickers are given to contributors
Flag Day in British English
noun
June 14, the annual holiday in the US to celebrate the adoption in 1777 of the Stars and Stripes
Flag Day in American English
US
1.
June 14, anniversary of the day in 1777 when the U.S. flag was adopted
2. [f- d-]
in England, any day when people give to some special fund for charity and get small flags in token of their contribution