as i said, you have to define the nested array first, this would be the right way to set up your array:
Code:
var aWeek:Array = new Array();
for(var z = 0; z<7; z++){
    var temp:Array = new Array();
    for(var j = 0; j<12; j++){
        temp.push("");
    }
    aWeek.push(temp);
}
zlatan