A simple script that waits for tickets to become available for the Harry Potter Studio Tour in London, and grabs them

WBTicketCheck.js

// Note that this has some limitations, such as looking specifically for adult tickets,
// looking for the given days only in the nearest month that has availability,
// and always choosing the earliest time if several are found within the desired dates.

function setAdultTickets(adultTicketsWanted) {
	const adultTicketsCount = parseInt($('.quantity-control.row > input')[0].value, 10);
	const ticketChangeIterations = Math.abs(adultTicketsWanted - adultTicketsCount);
	const ticketChangeButton = $(`.quantity-control.row > button.typcn-${adultTicketsCount < adultTicketsWanted ? 'plus' : 'minus'}`)[0];

	for (let i = 0; i < ticketChangeIterations; i++) {
		ticketChangeButton.click();
	}
}

function playSound(src) {
	return new Promise((resolve) => {
		const audio = new Audio(src);
		audio.onended = resolve;
		audio.play();
	});
}

function repeatHeyListen() {
	playSound('https://www.myinstants.com/media/sounds/hey_listen.mp3')
		.then(repeatHeyListen);
}

function waitForAvailability() {
	return new Promise((resolve) => {
		setTimeout(() => {
			const availableEls = $('.calendar>.row:not(.blankLoader) .calendar-body .day.available'), isLoading = $('.calendar-modal[data-component=eventTimeModal] .modal-content > .loading-mask.hide').length === 0;
			if (isLoading) {
				return waitForAvailability()
					.then((res) => resolve(res));
			}

			resolve(availableEls);
		}, 1000);
	});
}

function playSounds() {
	playSound('https://www.myinstants.com/media/sounds/mlg-airhorn.mp3')
		.then(() => playSound('https://www.myinstants.com/media/sounds/sound-9______.mp3'))
		.then(() => playSound('https://www.myinstants.com/media/sounds/ps_1.mp3'))
		.then(() => playSound('https://www.myinstants.com/media/sounds/wrong-answer-sound-effect.mp3'))
		.then(() => playSound('https://www.myinstants.com/media/sounds/lalalalala.swf.mp3'))
		.then(() => playSound('https://www.myinstants.com/media/sounds/tuturu_1.mp3'))
		.then(() => playSound('https://www.myinstants.com/media/sounds/hallelujahshort.swf.mp3'))
		.then(repeatHeyListen);
}

function addTicketsToBasket(dayElement) {
	dayElement.click();

	setTimeout(() => waitForAvailability()
		.then(() => {
			$('.ui-control.button.select-time')[0].click();

			setTimeout(() => {
				$('.typcn.typcn-shopping-cart.ng-binding')[0].click();
			}, 2000);
		}), 2000);
}

function checkForTickets(datesWanted=[6, 7, 8], adultTicketsWanted=2, checkFrequency=15) {
	setAdultTickets(adultTicketsWanted);

	function check() {
		$('.shared-calendar-button').click();

		waitForAvailability()
			.then(availableEls => {
				console.log(new Date(), 'Availability loaded. Checking for relevant dates...');
				for (let i = 0; i < availableEls.length; i++) {
					const day = parseInt(availableEls[i].innerText, 10);
					console.log('Day', day, 'is available...');
					if (datesWanted.includes(day)) {
						console.log('Found tickets!!!!!');
						playSounds();
						addTicketsToBasket(availableEls[i]);
						return;
					}
				}

				console.log(`Relevant dates not yet available. Will check again in ${checkFrequency} seconds.`);
				$('#page > div:nth-child(11) > div.modal.info-modal.w-auto-c > div > div.close').click();
				setTimeout(check, checkFrequency * 1000);
		});
	};
	check();
}

IIRC that's just how the website behaves - skips to the month that has the nearest available tickets - but as long as the script recognizes that it's the wrong month and instead of adding tickets, just keeps refreshing and waiting, then it should be fine.