Display a one time popover message when date selected

Well something along the lines of:

let displayed = false;
document.querySelector('select#special').addEventListener('click', function(event) {
    if (displayed === false) {
        // display your message...  
    }
});

I've written it in vanilla js, but it would be much easier to use some kind of framework. And for the message you could use this library which is pretty cool: https://sweetalert2.github.io/