// Obtem o ultimo dia do mês
var daysInMonth = new Date(year, month + 1, 0).getDate();

// Obtem primeiro dia da semana do mês
var firstWeekDay = new Date(year, month, 1).getDay();

Math.floor((daysInMonth + (weekday + firstWeekDay) % 7) / 7);

var count = 0,
    date = new Date(year, month, 1, 0, 0, 0);

while (month === date.getMonth()) {
    if (date.getDay() === weekday) count++;

    date.setDate(date.getDate() + 1);
}

var count = 0,
    increment = 1,
    date = new Date(year, month, 1, 0, 0, 0);

while (month === date.getMonth()) {
    if (date.getDay() === weekday) {
        count++;
        increment = 7;
    }

    date.setDate(date.getDate() + increment);
}