[j0hnik]

aleksmag,

document.addEventListener("DOMContentLoaded", function(){
		flatpickr(".flatpickr",{
			"disable": [
			function(date) {
            // return true to disable
           return (date.getDay() != 5 && date.getDay() != 6);
         }
        ],
         "locale": {
       "firstDayOfWeek": 1 // start week on Monday
     }
   });
});

вот так попробуйте