function formatDate(date) {

  var yyyy = date.getFullYear().toString();
  var mm = (date.getMonth()+1).toString(); // getMonth() is zero-based
  var dd  = date.getDate().toString();
  var data = (dd[1]?dd:"0"+dd[0]) + "/" + (mm[1]?mm:"0"+mm[0]) + "/" + yyyy;

  return data;

}

var calendarOptions = {
  language: "pt-BR",
  todayHighlight: true,
  datas: [],
  beforeShowDay: function(date) {
    date.setHours(date.getHours()+1);
    var data = formatDate(date);

    if(this.datas[data])
      return {
        "enabled": true,
        "tooltip": this.datas[data].title
      };
    else
      return {
        "enabled": false,
        "tooltip": "Não há sugestões de músicas para este dia ainda."
      };
  }
};

$(document).ready(function() {

   $.ajax({
          //url: 'https://musicasparamissa.com.br/datas.json',
          url: '/datas.json',
          crossDomain: true,
          dataType: 'json',
          type: 'GET',
          success: function( datas ) {

			   console.log('buscou datas', datas);

			   calendarOptions.datas = datas;

			   var onClick = function(e){
				 var date = e.date;
				 date.setHours(date.getHours()+1);
				 var data = formatDate(date);
				 var url = calendarOptions.datas[data].url;
				 //window.open(url);
				 document.location = url;
			   }

			   $('#calendar-navbar').datepicker(calendarOptions).on("changeDate",onClick);
			   $('#calendar').datepicker(calendarOptions).on("changeDate",onClick);
			   $('#calendar-footer').datepicker(calendarOptions).on("changeDate",onClick);

			}
      });

});
function formatDate(date) {

  var yyyy = date.getFullYear().toString();
  var mm = (date.getMonth()+1).toString(); // getMonth() is zero-based
  var dd  = date.getDate().toString();
  var data = (dd[1]?dd:"0"+dd[0]) + "/" + (mm[1]?mm:"0"+mm[0]) + "/" + yyyy;

  return data;

}

var calendarOptions = {
  language: "pt-BR",
  todayHighlight: true,
  datas: [],
  beforeShowDay: function(date) {
    date.setHours(date.getHours()+1);
    var data = formatDate(date);

    if(this.datas[data])
      return {
        "enabled": true,
        "tooltip": this.datas[data].title
      };
    else
      return {
        "enabled": false,
        "tooltip": "Não há sugestões de músicas para este dia ainda."
      };
  }
};

$(document).ready(function() {

   $.ajax({
          //url: 'https://musicasparamissa.com.br/datas.json',
          url: '/datas.json?v=2',
          crossDomain: true,
          dataType: 'json',
          type: 'GET',
          success: function( datas ) {

			   console.log('buscou datas', datas);

			   calendarOptions.datas = datas;

			   var onClick = function(e){
				 var date = e.date;
				 date.setHours(date.getHours()+1);
				 var data = formatDate(date);
				 var url = calendarOptions.datas[data].url;
				 //window.open(url);
				 document.location = url;
			   }

			   $('#calendar-navbar').datepicker(calendarOptions).on("changeDate",onClick);
			   $('#calendar').datepicker(calendarOptions).on("changeDate",onClick);
			   $('#calendar-footer').datepicker(calendarOptions).on("changeDate",onClick);

			}
      });

});
Update data.json version.