September 21, Independence Day

I'd like to send my greetings to the following countries, which are celebrating their respective independence days on this date:





Malta (1964, from the United Kingdom)





Belize (1981, from the United Kingdom)






Armenia (1991, from the extinct USSR)





It's curious to know that three different countries reached their respective independences on the same day, but different years. To what do we owe this strange coincidence? Is it that September 21 is a date that inspires sovereignity and indepentence spirit?

Cheers