JSFiddle - React, Tailwind, and code Playground

HTML

<table class="table-condensed calendar">

                        <thead>

                            <tr class="dotw">
                                <td>ПН</td>
                                <td>ВТ</td>
                                <td>СР</td>
                                <td>ЧТ</td>
                                <td>ПТ</td>
                                <td>СБ</td>
                                <td>ВС</td>
                            </tr>
                        </thead>
                        <tbody>
                            <tr>
                                <td class="off-date">31</td>
                                <td>1</td>
                                <td>2</td>
                                <td>3</td>
                                <td>4</td>
                                <td class="event-number">5</td>
                                <td>6</td>
                            </tr>
                            <tr class="event-text">
                                <td></td>
                                <td></td>
                                <td></td>
                                <td></td>
                                <td></td>
                                <td class="event-background">Концерт в
                                    <br> омске</td>
                                <td></td>
                            </tr>
                            <tr>
                                <td>7</td>
                                <td>8</td>
                                <td>9</td>
                                <td>10</td>
                                <td>11</td>
                                <td>12</td>
                                <td>13</td>
                            </tr>
                            <tr class="event-text">
                                <td></td>
                                <td></td>
                                <td></td>
                                <td></td>
         ...

CSS

.calendar tbody {
    position: relative;
}

.event-number {
    background: #134699;
    color: #fff;
}
.event-background {
    position: absolute;
    display: none;
}

JavaScript

$(document).ready(function(){
            $(".event-number").click(function(){
                $(".event-background").slideToggle("slow");
                return false;
            });
        });