how to display automatically the month in master title

Fun with ExtendScript:

var nextSunday;

nextSunday = getNextSunday ();

alert (nextSunday.getMonth()); // 0 = Jan, 1 = Feb, etc.
alert (nextSunday.getDate());
alert (nextSunday.getFullYear());

function getNextSunday () {

    var today, currentDay, daysTilSunday, sundayDate;

    today = new Date();
    currentDay = today.getDay(); 

    daysTilSunday = (7 - currentDay) % 7;

    sundayDate = new Date();
    sundayDate.setDate(today.getDate() + daysTilSunday); 

    return sundayDate;
}

This could be developed into a script to solve the original poster’s issue, but I wouldn’t be able to do it for free.

If your localization settings correctly render the short month name, perhaps insert System Variable
Current Date (short)
and edit its Definition to:
<$daynum> <$shortmonthname>

Depending on your intent, there are several other similar date variables you might use instead.