JSFiddle - React, Tailwind, and code Playground

HTML

<table>
<tr class="row">
<td>day 1</td>
<td>day 2</td>
<td>day 3</td>
<td>day 4</td>
<td class="today">day 5</td>
<td>day 6</td>
<td>day 7</td>
<td>day 8</td>
</tr>
</table>

CSS

td {
    margin: 0;
    padding: 10px;
    border: 1px solid #ccc;
}

.selected {
    background: blue;
    color: white;
}

JavaScript

$('td').on('mouseenter', function(){
    var todayIndex = $(this).siblings('.today').index(),
        index      = $(this).index(),
        fromTo     = index < todayIndex ? [index, todayIndex + 1] : [todayIndex, index + 1];

    $("td")
        .removeClass("selected")
        .slice.apply($("td"), fromTo)
        .addClass('selected');
});