Денис Инешин

$('.calendar').on('click', '.day', function () {
    var day = $(this).data('date');
    console.log(day);
});

var $btn = $('#toggle'),
    $popup = $('.popup'),
    $test = $('.test'),
    visible = false;

function togglePopup () {
    visible = !visible;
    
    $test.toggleClass('hidden', visible);
    $popup.toggleClass('hidden', !visible);
}

$btn.on('click', togglePopup);