Date function in Javascript

I want to get the week day from day in a month.

const lastDay = new Date(date.getFullYear(), date.getMonth()+1, 0).getDate();
let available_days = [2,3,5];

for(let i =1; i <= lastDay; i++){
    let x = i % 7;
	// i want to get the weekday from i which is the day
    // let Array = [0,3,5];
    if(available_days.indexOf(x) >= 0){
        days += `<button style="padding:6px;" class="hover:bg-[#126376] hover:text-white transition duration-150 ease-in bg-[#BDF0FC] rounded-full">${i}</button>`;
    } else {
        days += `<button style="padding:6px;" class="bg-gray-400 rounded-full" disabled>${i}</button>`;
    }
}

how can i get it. this is how it looks https://ibb.co/pfBNXKb