Design a fantasy football schedule: 12 teams, 3 divisions

I like the part about beginning and ending with own division. It is not for me to say that I dislike missing a Bye week where a 'double header' is included. Revising the last comment, where the six targeted weeks cover the six intradivision games (a minimum),  there are seven weeks to cover all but one of the eight inter division games. Remaining are the last inter-division game and the other end of the doubeheader.

> 7 games with other divisions during weeks 4 - 10, so the 8th interdivision game must be during week 13 as part of the 'doubleheader'.

Everybody gets to take week 14 off, before thinking about any potential playoffs. How to break any ties is another matter entirely.

That contradicts
> Intradivision games where teams only play other teams within their division must be in weeks 1, 2, 3 and 11, 12, 13.

@d1=(1..4);
@d2=(5..8);
@d3=(9..12);
$\=$/;
print "Intradivision weeks:";
for( 1..2 ){
  for( 1..3 ){
    for ( \@d1,\@d2,\@d3 ){
        print "team $_->[0] vs team $_->[1]";
        print "team $_->[2] vs team $_->[3]";
        @$_ = @$_[0,2,3,1]
    }
    print '';
 }
}
print "Interdivision weeks:";
for (1..8){
    print "team $d1[3] vs team $d2[0]";
    print "team $d1[2] vs team $d2[1]";
    print "team $d2[3] vs team $d3[0]";
    print "team $d2[2] vs team $d3[1]";
    print "team $d3[3] vs team $d1[0]";
    print "team $d2[2] vs team $d1[1]";
    (@d2[1..3,0],@d3)=(@d3,@d2);
    print '';
}
__END__
produces the following
Intradivision weeks:
team 1 vs team 2
team 3 vs team 4
team 5 vs team 6
team 7 vs team 8
team 9 vs team 10
team 11 vs team 12

team 1 vs team 3
team 4 vs team 2
team 5 vs team 7
team 8 vs team 6
team 9 vs team 11
team 12 vs team 10

team 1 vs team 4
team 2 vs team 3
team 5 vs team 8
team 6 vs team 7
team 9 vs team 12
team 10 vs team 11

team 1 vs team 2
team 3 vs team 4
team 5 vs team 6
team 7 vs team 8
team 9 vs team 10
team 11 vs team 12

team 1 vs team 3
team 4 vs team 2
team 5 vs team 7
team 8 vs team 6
team 9 vs team 11
team 12 vs team 10

team 1 vs team 4
team 2 vs team 3
team 5 vs team 8
team 6 vs team 7
team 9 vs team 12
team 10 vs team 11

Interdivision weeks:
team 4 vs team 5
team 3 vs team 6
team 8 vs team 9
team 7 vs team 10
team 12 vs team 1
team 7 vs team 2

team 4 vs team 12
team 3 vs team 9
team 11 vs team 5
team 10 vs team 6
team 8 vs team 1
team 10 vs team 2

team 4 vs team 8
team 3 vs team 5
team 7 vs team 12
team 6 vs team 9
team 11 vs team 1
team 6 vs team 2

team 4 vs team 11
team 3 vs team 12
team 10 vs team 8
team 9 vs team 5
team 7 vs team 1
team 9 vs team 2

team 4 vs team 7
team 3 vs team 8
team 6 vs team 11
team 5 vs team 12
team 10 vs team 1
team 5 vs team 2

team 4 vs team 10
team 3 vs team 11
team 9 vs team 7
team 12 vs team 8
team 6 vs team 1
team 12 vs team 2

team 4 vs team 6
team 3 vs team 7
team 5 vs team 10
team 8 vs team 11
team 9 vs team 1
team 8 vs team 2

team 4 vs team 9
team 3 vs team 10
team 12 vs team 6
team 11 vs team 7
team 5 vs team 1
team 11 vs team 2
How you want to number the weeks may depend on how you want to resolve the contradiction

ozo,

in nearly all the inter-division groups there's a team playing 2 games the same day. e.g. the last one has 11 playing 2 and 7, with no game for 8.

Too many $d2[2] cells?

Sorry
print "team $d2[2] vs team $d1[1]";
should have been
print "team $d3[2] vs team $d1[1]";

The intradivisional games are weeks 1, 2, 3 and 11, 12, 13.
One of the week 13 double header game will be an out of division game.