Commemoration of Hungary’s first king St. Stephen, also the day of the foundation of Hungary and “the day of the new bread”. St. Stephen (Szent István király, ca. 975 – 15 August 1038), as the first king of Hungary, led the country into the Christian church and established the institutions of the kingdom and the church. He was canonized on 20 August 1083, and 20 August is his feast day in Hungary.

Celebrated with a half-hour fireworks display on the bank of the Danube in the evening, watched from both river banks, the Buda hills, and the rooftops of Pest and Buda. 

https://en.wikipedia.org/wiki/Public_holidays_in_Hungary