128 years? Hypothetical leap year reform
C (gcc), 60 bytes



f(m,d,y){y-=m&lt;3;return(y+y/4-y/128+"-bed=pen+mad."[m]+d)%7;}


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Simple modification of Sakamoto's method. Takes input as integer arguments in the order month, day, year, and outputs the number of the day (0-indexed on Sunday).