December 18th 1979. After approximately a decade as leader of the Liberal Party and prime minister of Canada over three consecutive terms, Pierre Trudeau resigned from politics after losing to Joe Clark's PCs in the general election of 1979. But before the year was over, so was Clark's government, and Trudeau came back to fight a fifth and last election.
A second news article in this clip, from February 1980, gives an overview of Trudeau's career upon his comeback.