"Vidovdan" of your national history

Which day periodically appears at your national history?

To write this thread inspired me story of VIDOVDAN (St. Vitus Day) at Serbian history,
as well as the fact that 2 Czech kings were killed in the battle at the same day
(St. Rufus Day, i.e. on 26 August):

Battle of Marchfeld - on 26 August 1278 was in the battle killed Přemysl Ottokar II, King of Bohemia
Battle of Crécy - on 26 August 1346 was in the battle killed John the Blind, King of Bohemia

On 26 August 1992 - Václav Klaus and Vladimír Mečiar signed agreement about dissolution of Czechoslovakia.


Do you know about some important day what appears in your national history?

Unfortunatelly none date comes to my mind tbh, but I guess another Polish members will be able to answer