SO - 19949400

HTML

<div class="message" message-id="1">a</div>
<div class="message" message-id="1">a</div>
<div class="message" message-id="2">a</div>
<div class="message" message-id="2">a</div>
<div class="message" message-id="3">a</div>

CSS

.group {
    border: 1px solid red;
    margin-bottom: 5px;
}

JavaScript

var groups = {};
$('.day').each(function () {
    var className = this.className.match(/(day\d+)/)[1],
        $group = groups[className];
    if (!$group) {
        $group = $('<div />', {
            'class': 'group'
        }).insertAfter(this);
        groups[className] = $group;
    }
    $group.append(this);
})