1function sundaysInMonth( m, y ) {
2  var days = new Date( y,m,0 ).getDate();
3  var sundays = [ 8 - (new Date( m +'/01/'+ y ).getDay()) ];
4  for ( var i = sundays[0] + 7; i < days; i += 7 ) {
5    sundays.push( i );
6  }
7  return sundays;
8}
9
10alert( sundaysInMonth( 10,2012 ) ); //=> [ 7,14,21,28 ]
11alert( sundaysInMonth( 10,2012 ).length ); //=> 4

1function mondaysInMonth(m,y) {
2  var days = new Date(y,m,0).getDate();
3  var mondays =  new Date(m +'/01/'+ y).getDay();
4  if(mondays != 1){
5    mondays = 9 - mondays;
6  }
7  mondays = [mondays];
8  //console.log(mondays);
9  for (var i = mondays[0] + 7; i <= days; i += 7) {
10    mondays.push(i);
11  }
12  return mondays;
13}