On June 28, 1987 One day after hitting three home runs in Oakland’s 13 – 3 rout of the Indians, A’s rookie 1B Mark McGwire hits two more in a 10 – 0 Oakland romp to tie the major-league record of five homers in two games. McGwire’s nine runs in two games ties an American League record set in 1937.
Source
Baseball Reference June 28