Group ACF repeater fields from differrent posts

Start by collecting all the dates of the current post (event) in the array. Then iterate throught the array and get all events for each date.

// all dates from current event in format "Ymd"
$all_dates = [];
for ($i = 0; $i < $date_count; $i++) {

    // Get repeater fields 
    $dates="dates_times_" . $i . '_date';
    $temp_date = get_post_meta( get_the_ID(), $dates, true );

    // if value of dates_times_XX_date is in format 'Ymd' conversion is not needed

    // Group by day
    if ( !isset( $all_dates[$temp_date] ) )
        $all_dates[] = $temp_date;
}
if ( !empty($all_dates) )
    sort($all_dates);

foreach ($all_dates as $l_date) {

    $query = "select post_id from {$wpdb->postmeta} where meta_key like 'dates_times_%_date' and meta_value=%s";

    // IDs of events with that date
    $events = $wpdb->query( $wpdb->prepare($query, $l_date) );

    // get events with IDs, 
    // display header '<h2 class="medium-12 cell"> date </h2>';
    // display events 
    // REST OF POST CODE HERE
}