When was Antarctica formally discovered by a European expedition?

A Russian expedition led by Fabian Gottlieb Thaddeus von Bellingshausen (captain of the 'Vostok') and Mikhail Petrovich Lazarev (commanding the 2nd ship the 'Mirny') successfully circumnavigated Antarctica as well as discovered and mapped numerous islands during their 751-day expedition.

Beginning in April 1819, the two-ship expedition left the Russian port of Kronstadt bound for the Southern Ocean to either find the presumed southern continent or disprove its existence. In January 1820, at coordinates 69° 21' 28" South, 2° 14' 50" West, land was spotted by the expedition. Land was again spotted in February. The route taken by the expedition circumnavigated the continent allowed for mapping of other islands, such as Peter I Island, Alexander Island, Elephant Island and others. The expedition returned to Kronstadt in 1821 and established the existence of the southern continent.

The 2nd circumnavigation journey of James Cook in 1774 approached Antarctica as close as 71° South but did not see land and they were unable to travel further due to ice.