Why i get "undefined" from JavaScript Ajax return function?

Question

I'm trying to return an array from the function bellow, but it always returns "undefined". But when i do console.log() inside the function there are results.

Function:

function getDaysWithEvents( month, year ) {
    let _token = $('meta[name="csrf-token"]').attr('content');
    $.ajaxSetup({ headers: {'X-CSRF-TOKEN': $('meta[name="csrf-token"]').attr('content')}});
    $.ajax({
        url: "/agenda/checkDaysW",
        type: "GET",
        data: {
            month:month,
            year:year,
            _token: _token,
        },
        success: function ( eventDays ) {
            console.log ( eventDays );  /*  --- results OK  -- */
            return eventDays;
        },
    });
}

How i am calling the funtion:

var todayDate = new Date(); 
eventArray = getDaysWithEvents( todayDate.getMonth()+1, todayDate.getFullYear() );

This returns "undifined". What am i doing wrong ?