The June 28, 1914 assasination of Franz Ferdinand started World War One. A war in which brave little Serbia sparked loads of sympathy in London.
Allied support led to the dismantling of the Habsburg Empire and the formation of the Kingdom of Yugoslavia. The new kingdom got its first constitution signed on June 28, 1921.
The mother of all June 28’s was, of course, the mentioned battle on Kosovo Polje, in which the Serbs where decimated by the Ottomans – all in quite a heroic way. And the Ottoman Sultan was killed as well.
On the exact spot, on June 28 1987, Milosevic started playing the nationalist card.
And from there, it was not a huge step to ethnic cleansing. A very nasty war later, Milosevic was brought to justice on June 28, 2001.