Short answers, pulled from the story.
The Pan-European Picnic was a peace demonstration held on the 19th of August 1989 on the Austro-Hungarian border near Sopron, Hungary. During the event, several hundred East German citizens crossed through an opened border gate into Austria, making it the largest mass exodus from East Germany since the Berlin Wall was built in 1961.
The picnic was organised on the ground by four Hungarian opposition parties: the Hungarian Democratic Forum, the Alliance of Free Democrats, Fidesz, and the Independent Smallholders, Agrarian Workers and Civic Party. Its patrons were Otto von Habsburg and Hungarian Minister of State Imre Pozsgay. The original idea came from a conversation between Habsburg and two MDF representatives, Mária Filep and Ferenc Mészáros, at a dinner in Debrecen on the 20th of June 1989.
On the day of the picnic, 661 people crossed the border. Many thousands more waited nearby but were uncertain whether the crossing was truly safe. On the 11th of September 1989, after Hungary formally opened its border, 30,000 East Germans fled to the West.
Otto von Habsburg was one of the two patrons of the event and, along with Ferenc Mészáros, was credited with originating the idea of opening the border as a way to test Mikhail Gorbachev's reaction. He was not present on the day; his daughter Walburga von Habsburg attended in his place. After the fall of the Berlin Wall, Erich Honecker said of Habsburg: "this Habsburg drove the nail into my coffin."
Hungarian Prime Minister Miklós Németh faced a crisis: roughly 100,000 East Germans were stranded in Hungary that summer and could neither be kept there through winter nor returned without subjecting them to Stasi persecution. Németh ordered border troops not to intervene if East Germans crossed, seeing the picnic as an opportunity to gauge whether the Soviet Union would respond. No call came from Moscow.
On the 11th of September 1989 Hungary fully opened its western border and 30,000 East Germans fled to the West. Erich Honecker was relieved of his position on the 17th of October. The Berlin Wall fell on the 9th of November 1989. The Warsaw Pact disintegrated in 1991, and Germany was reunified.