This also works with years:

beforeShowDay: function(date) {
var day = date.getDay();

function IsClosed(date) {

var day = date.getDate();
var month = date.getMonth() + 1;
var year = date.getFullYear();

return (day === 25 && month === 12 && year === 2023)+(day === 24 && month === 12 && year === 2024);
}
return [(!IsClosed(date))];
},