Is a big deal; a holiday to remember the uprising of the mid 1840s when Hungarian leaders led people in revolt against the Austrians (a.k.a. Hapsburgs). You can learn a little more about this big day HERE. You can learn a lot more in this (contested for objectivity) article on Wiki (ALL history from the whole region -not just Hungary- that I have read and been told as historical is contestable due to sometimes lack of objectivity – you know, “the South shall rise again” and all that).
Nonetheless, it is an important remembrance and important time for my beloved Hungarian friends to recreate as well as remember.
kingdom travelin' 
Thanks for remembering and thanks for making remember!