Short answers, pulled from the story.
German reunification officially occurred at 00:00 CEST on the 3rd of October 1990, when the German Democratic Republic ceased to exist and its five reconstituted states joined the Federal Republic of Germany. The 3rd of October has since been celebrated as German Unity Day, the country's national holiday.
The opening of a border gate between Austria and Hungary at the Pan-European Picnic on the 19th of August 1989 triggered the process. The Austrian branch of the Paneuropean Union, headed by Karl von Habsburg, distributed thousands of brochures inviting East German tourists in Hungary to a picnic near Sopron, from which they escaped to the West. By the end of September 1989, more than 30,000 East German citizens had fled west through Hungary.
British Prime Minister Margaret Thatcher and French President Francois Mitterrand were among the most prominent opponents. Thatcher feared Germany's size, central location, and "national character" would make it a destabilising force in Europe and favoured a five-year transition period. A January 1990 poll found that 69 percent of Poles and 50 percent of French and British respondents worried about a reunified Germany becoming the dominant power in Europe.
On the 18th of May 1990, the two German states signed a treaty on monetary, economic, and social union, which came into force on the 1st of July 1990. The West German Deutsche Mark replaced the East German mark as the official currency of East Germany. West Germany also began paying subsidies into the East German budget and social security system, and many West German laws came into force in the GDR ahead of full political unification.
Mauer im Kopf, meaning "wall in the head," described the psychological divide between East and West Germans that persisted after the physical Berlin Wall came down. A 1993 survey by the Allensbach Institute found that only 22 percent of West Germans and 11 percent of East Germans felt they were one nation. Economic disparities, cultural differences built over more than 30 years of separation, and resentment over privatisation policies all contributed to the divide.
The 9th of November 1989 was the date the Berlin Wall fell, but it coincided with the anniversary of Kristallnacht, the first large-scale Nazi-led pogroms against Jews in 1938, making it considered inappropriate as a national holiday. The 3rd of October 1990, the date of formal reunification, was chosen instead as the German Unity Day.