Pikaday: Enable one specific date, Sunday

I would like to enable one specific date, which is a Sunday. Currently all Sundays are blocked.

I added this code at the end of 'product.liquid'.

{{ 'moment.min.js' | asset_url | script_tag }}      
{{ 'pikaday.js' | asset_url | script_tag }}
{{ 'pikaday.jquery.js' | asset_url | script_tag }}
<script>
var picker = new Pikaday({ 
   disableDayFn: function(date){
    // Disable Sunday, Enable Specific Sunday
     return date.getDay() === 0 && date.getDate() != "05/14/2017";
},
    field: $('#del_date')[0],
    format: 'MM/DD/YYYY',
    onSelect: function() {
        console.log(this.getMoment().format('Do MMMM YYYY'));
    }    
});

Check disableDayFn to disable particular date inside Pikaday

Fiddle demo

var start_date = new Pikaday({
  disableDayFn: function(date) {
    var enabled_dates = ["05/14/2017"]; // dates I want to enabled.
    if (date.getDay() === 0 && $.inArray(moment(date).format("MM/DD/YYYY"), enabled_dates) === -1) {
      return date;
    }
  },
  format: 'MM/DD/YYYY',
  field: document.getElementById('start_date'),

})

;