20 August is the national holiday in Hungary, the day of the state-founder and first king of the country Stephan the Saint (Szent István). He was sainted on 20 August 1083, hence the tradition to have this celebration.
The festivities that start on August 18 with the traditional Arts and Crafts festival held in the Buda Castle include a concert in front of the Parliament, international bread festival at Vajdahunyad Castle in the City Park on August 19, and fireworks on August 20.
According to The Budapest Sun, this year the government is to spend Ft208 million ($1 million) net on the fireworks and other events of this national holiday.
The fireworks ceremony will start on Sunday at 9 PM and take about half an hour. The rockets are fired from the western side of the Danube, Gellérthegy, and thus the best viewpoint would be Pest bank of the river or one of the central bridges (Lánchíd, Margit híd).