JSFiddle - React, Tailwind, and code Playground

JavaScript

// find all dates that fall on a particular day of the week
// for the given year and month

function getDaysOfMonth(year, month, dow) {
    --month;                                    // to correct for JS date functions
    var d = new Date(year, month, 1);           // get the first of the month
    var dow_first = d.getDay();                 // find out what DoW that was 
    var  date = (7 + dow - dow_first) % 7 + 1;  // and hence the first day matching dow
    
    var dates = [];
    d.setDate(date);
    do {
        dates.push(new Date(d));      // store a copy of that date
        date += 7;                    // go forward a week
        d.setDate(date);            
    } while (d.getMonth() === month); // until the end of the month
    
    return dates;
}

document.write(getDaysOfMonth(2011, 12, 5).join("<br>"));