Birthday? Ah, you're referring to the treaty of London of 1839, however, that treaty is not a declaration of independence, just the Dutch giving in after 9 years of refusing the reality of Belgian independence and ratifying a treaty that was written in 1831. So, it's not the birthday of Belgium at all, for two reasons: you can't have your first birthday when you're almost ten and the Belgian revolution began in August and October the Provisional Government declared independence on October 4th, so you missed it by a couple of months. But don't worry Belgium doesn't celebrate that day, neither is the day in April celebrated [The treaty meant the loss of territories that had joined the Belgian Revolution, except Zeelandic Flanders which was conquered, so it's not such a happy memory].
Ah, I saw. Well, it's kind of like America. America didn't truly become independent on July 4th; it just declared its independence. We still had to fight to break away from Great Britain's rule, but we still celebrate the 4th of July.
Oh!~ Thank you do much, GydroZMaa-chan!~ (Aha sorry force of habbit to RP when it has something to do with Belgium/Belarus because in my group of friends they made me those two people, Ironic, isn't it?)