try this
eventAfterAllRender: function () {
var map ={Sun:'City1',Mon:'City2',Tue:'City3',Wed:'City4',Thu:'City5',Fri:'City6',Sat:'City7'};
$('.fc-widget-header th').each(function (i,elem) {
var newText = map[$(elem).text()];
$(elem).text(newText);
});
}