does anyone know offhand if there are any days in which there's an episode from all 5 years?
It certainly seems unlikely. I've only mapped out the eps through the end of August, but already there's a discernible pattern to the week:
Sundays: 4 eps (1966 was also a Sunday)
Mondays: 4 eps (1970 was a Saturday)
Tuesdays: 3 eps (1969 was a Saturday; 1970 was a Sunday)
Wednesdays: 3 eps (1968 was a Saturday; 1969 was a Sunday)
Thursdays: 4 eps (1968 was a Sunday)
Fridays: 4 eps (1967 was a Saturday)
Saturdays: 3 eps (1966 was also a Saturday; 1967 was a Sunday)
Though, of course, that's barring pre-emptions.
1968 was a leap year, so the pattern will change for March, April, May and the first three weeks of June. (I'm assuming it should shift to S:4, M:4, T:3, W:3, T:3, F:4, S:4 for March 1st through April 2nd and then to S:3, M:4, T:3, W:2, T:2, F:3, S:3 thereafter.)