Вот че нам препод один ответил на такой же вопрос:

15 March, 1848 was the most special day in Hungarian history. It was a bloodless revolution. On that day about a dozen of young guys (most of them younger than you) developed a uniquely successful framing strategy. They gathered in the café called Pilvax, not far from CEU, in the morning and summarized in 12 points the aims of the would-be revolution (freedom of press, equality of rights, end of serfdom, equal taxation, etc.). Petofi, the poet, has read a new poem in which he asked the nation whether “Shall we be free or shall we be slaves?”. Anticipating the answer to this particularly complex question he included a refrain “We swear to the God of Hungarians, We will not be slaves any longer”. Then he was touring the city with the poem, and by afternoon everybody was “swearing” in Budapest. They went then to a local publisher to print the leaflets with the 12 points. The publisher was loudly protesting, requiring the permission of the censor. Then he whispered to the youngsters: “Confiscate my machines. I will have no choice then.” After some second the revolutionaries understood what the publisher wants and “in the name of the people” they confiscated the machines. Then the publisher was happily printing the slogans.

Covering the city with leaflets and making fiery speeches the youngsters attracted a huge crowd. (Though for lunch most of them went home.) The crowd was largely composed by peasants since on that day there was a big market in the city. The city leadership was convinced that under the large coats of the peasants weapons are hidden and when the youngsters demanded the release of political prisoners and the acceptance of the revolutionary demands, none of the authorities dared to resist. The release of the prisoners turned out to be not as spectacular as planned since they found only one. But the acceptance of the demands make the day the most progressive day in Hungarian history (ever since).

The revolution has liberal, nationalist, and (less plausible) socialist reading. But the reason for the separate celebrations is different: Hungarian parties do not celebrate together anything. They all claim that if Petofi was alive he would not celebrate either with the others.