October 20: Birth of the Báb, a holy day in the Bahá'í Faith.

• 1740 – Maria Theresa (pictured) assumed the throne of the Habsburg Monarchy in Austria, following the Pragmatic Sanction of 1713.

• 1818 – The United Kingdom and the United States signed the Treaty of 1818, which settled the Canada–United States border on the 49th parallel for most of its length.

• 1827 – An allied British, French, and Russian naval force destroyed a combined Turkish and Egyptian fleet at the Battle of Navarino, a decisive moment in the Greek War of Independence.

• 1941 – World War II: German soldiers began a massacre of thousands of civilians in Kragujevac in Nazi-occupied Serbia.

• 1973 – Queen Elizabeth II, in her capacity as Queen of Australia, formally opened the Sydney Opera House on Bennelong Point in Sydney Harbour.

More events on this day... Recent days: October 19 – October 18 – October 17