On this day in 1573, Henryk Walezy (Henry of Anjou) is sworn into office after he became the first elected king of the Polish- Lithuanian commonwealth, and the first ever head of state that was voted into office in a general elections.
The next head of state in history to be voted into office in a popular elections would be George Washington, and that event would take place 216 years after the first Polish- Lithuanian election.
In fact, there was even a higher voters tunrout at the Polish-lithuanian elections, where only nobles could vote (around 40,000 people) than at the first US elctions, where only white male land owenrs - the US version of nobility - were allowed to vote.
Only two years after he was voted to office, Henryk Walezy was also the first democratically elected head of state which was fired by his parliament for basically being a failure as a king.
Despite this first failure, the tradition of democratically elected kings survived in the Polish-Lithuanian commonwealth up until 1795, when the Commonwealth's despotic and autocratic neighbors destroyed it.
Source : http://www.amazon.com/Polish-Way-Thousand-Year-History-Culture/dp/0781802008