This is a really fascinating article. It's always interesting to hear these sort of humanistic anecdotes about Bo.
One More Time: Why This Game is Special
Was the Michigan/Michigan State game, in Bo's era, played with greater frequency on the second Saturday in October? Because of late, it very much has the feel of a moveable feast (which is not a bad thing), but the article asserts it as a fact when it's really more of a "fact".
Yes, this was the traditional date for the Michigan-Michigan State game for decades, with some small variance.
During Bo's tenure--
First Saturday in October: 1979, 1984 (2 games)
Second Saturday in October: 1971-1978; 1980-1983; 1985-1989 (17 games)
Third Saturday in October: 1969-1970 (2 games)
The Michigan-Michigan State game was never earlier than October 6, never later than October 18 from 1961 to 1994.
That is a great bit of research (well appreciated) and I kind of wish that perhaps the Big Ten would consider moving it back to the second week in October date as a "division rivals" day or some such.