|St. Stephen I, King of Hungary|
First, it is the anniversary of the founding of Hungary as a kingdom in 1000 CE.
Originally it was just a principality in the Carpathian Basin of Central Europe, established just prior to 900 CE. The Hungarians were semi-nomadic, pushing westward in their search for more territory. The westward push was halted by Holy Roman Emperor Otto I at the Battle of Lechfeld* on 10 August 955. This convinced the Hungarians to settle down where they were.
There was internal strife in Hungary, however. When the ruling probe of Hungary, Géza, died, his brother Prince Koppány claimed the right to rule. This claim was challenged by Géza's son, Vajk, who had been baptized a Christian and taken the name István (Stephen). Stephen wanted to turn Hungary into a European Christian kingdom. Koppány wanted to adhere to the traditional tribal ways. Forces loyal to each clashed in 998 where Stephen, whose Christian connection brought him the support of Bavaria, prevailed. Koppány was cut into 4 pieces and the parts sent throughout the kingdom as a sign of how troublemakers could expect to end up.
On Christmas Day 1000, Stephen I was crowned King of Hungary with a crown sent to him by Pope Sylvester II, establishing Hungary as a kingdom and country in its own right. Stephen turned out to be a great friend to the Roman Catholic Church, which leads us to the second reason this date is important for Hungary.
In 1083, Stephen I of Hungary was canonized as St. Stephen. He established the presence of the church in Hungary very throughly, with six bishoprics and three monasteries and penalties for not following Christianity. Hungary became a safe route for pilgrims traveling from Western Europe to the Holy Land.
Sadly, no children by him and his queen, Gisela of Bavaria (also known as Gisela of Hungary), survived, and upon his death there was civil war. His reign was a time of peace and a golden age for the spread of Christianity in Central Europe. His feast day, 20 August, is also a holiday in Hungary celebrating the founding of the country.
*Curiously, this was the second Battle pif Lechfeld; in the first, fought in 910, Hungarians defeated a Frankish army, but failed to establish dominance in the region.