When was the date of the Electoral College meeting established by law and how often was the meeting not held on that date?
From Wikipedia
Since 1936, the date fixed by Congress for the meeting of the Electoral College is "on the first Monday after the second Wednesday in December next following their appointment".
The Electoral College has met every four years since the first meeting. The current date was changed in 1936 due to the Twentieth Amendment setting the start of presidential terms to January 20th.
The first meeting date was decided by the Continental Congress, because members of the first US Congress had not been selected. The dates for subsequent meetings were then established by law. Later, the dates were changed three times, but never, seemingly, to accommodate a contested election.
The dates of meetings of the Electoral College for Maryland are given here, Archives of Maryland Historical List Presidential Electors in Maryland, 1789-. It is clear that the fixed dates, except the first meeting, were changed occasionally in different periods.
Wednesday, February 4, 1789
Wednesday, December 5, 1792
Wednesday, December 7, 1796
Wednesday, December 3, 1800
Wednesday, December 5, 1804
Wednesday, December 7, 1808
Wednesday, December 2, 1812
Wednesday, December 4, 1816
Wednesday, December 6, 1820
Wednesday, December 1, 1824
Wednesday, December 3, 1828
Wednesday, December 5, 1832
Wednesday, December 7, 1836
Wednesday, December 2, 1840
Wednesday, December 4, 1844
Wednesday, December 6, 1848
Wednesday, December 1, 1852
Wednesday, December 3, 1856
Wednesday, December 5, 1860
Wednesday, December 7, 1864
Wednesday, December 7, 1868
Wednesday, December 4, 1872
Wednesday, December 6, 1876
Wednesday, December 1, 1880
Wednesday, December 3, 1884
Monday, January 14, 1889
Monday, January 9, 1893
Monday, January 11, 1897
Monday, January 14, 1901
Monday, January 9, 1905
Monday, January 11, 1909
Monday, January 13, 1913
Monday, January 8, 1917
Monday, January 10, 1921
Monday, January 10, 1925
Wednesday, January 2, 1929
Wednesday, January 4, 1933
Monday, December 14, 1936
Monday, December 16, 1940
Monday, December 18, 1944
Monday, December 13, 1948
Monday, December 15, 1952
Monday, December 17, 1956
Monday, December 19, 1960
Monday, December 14, 1964
Monday, December 16, 1968
Monday, December 18, 1972
Monday, December 13, 1976
Monday, December 15, 1980
Monday, December 17, 1984
Monday, December 19, 1988
Monday, December 14, 1992
Monday, December 16, 1996
Monday, December 18, 2000
Monday, December 13, 2004
Monday, December 15, 2008
Monday, December 17, 2012
Monday, December 19, 2016