"History of Serbs and Serbia" Republic of Serbia by PiceaOmorika

The oldest known description of a flag of Serbia is from 1281 description of treasury of king Stefan Vladislav, which was kept in Dubrovnik Republic. The description lists vexillum unum de zendato rubeo et blavo - a flag of fabric red and blue.
As Vladislav ruled from 1234 to 1243 and died after 1264, the flag was used earlier than it was described, around the middle XIII century.

Flag of Mighty Yugoslavia 1945-1991

Serbia (Yugoslavia) can be viewed as a collection of disparate ethnic groups who are largely hostile to one another, following a long history of conflict. Though Serbia had been independent at various points in time, the Ottoman conquest in 1389 kept Serbia submissive until 1878, when it reestablished its independence. The Balkans engaged in a series of wars, the last of which led directly to the outbreak of World War I. After the war, Yugoslavia was cobbled together from many small nations and peoples: Serbs, Slovenes, Croats, Bulgarians, Albanians, Macedonians, and more. The uneasy alliance shredded with the coming of the Nazis in World War II as some looked to Germany as liberators and others assumed an anti-German posture. The anti-forces were led by Josip Broz Tito. Though he was victorious, the war had cost millions their lives and left the country in shambles. The federal People's Republic of Yugoslavia endured while Tito lived, thanks to his iron control of the country, and for a short time after. His death in 1980 was not followed by immediate trouble -- it took the fall of communism in 1989 to destabilize the country. At that point, only Serbia and Montenegro voted for Communist rule, while Croatia, Slovenia, Macedonia, and Bosnia-Herzegovina chose the route to independence. The violent civil war that ensued shocked the world and required the efforts of the United Nations and NATO to bring to a still-uncertain conclusion.

Saint Sava

Saint Sava (Serbian-Свети Сава) (1169 - January 14, 1236), originally the prince Rastko Nemanjić (son of the Serbian ruler and founder of the Serbian medieval state Stefan Nemanja and brother of Stefan Prvovenčani, first Serbian king), is the first Serb archbishop (1219-1233), the most important saint in the Serbian Orthodox Church and important cultural and political worker of that time.

In his youth, he fled from his home to join the orthodox monastic colony on Mount Athos (Holy Mountain on the Chalkidiki peninsula) and was given the name Sava. He first traveled to a Russian monastery and then moved to the Greek Monastery of Vatopedi. At the end of 1197 his father, king Stefan Nemanja joined him. In 1198 they together moved to and restored the abandoned monastery Hilandar, which was at that time the center of Serbian Christian monastic life.