GroupBy custom week in Collection

Have a collection and want to group the results by week starting on Wednesday.

Have this, but return the key correctly, but not the results:

$all_days = $all_days->groupBy(function($date) {
    $event_start = Carbon::parse($date->event_start);
    $start = $event_start->startOfWeek()->addDays(2)->format('Y-m-d');
    $end = $event_start->endOfWeek()->addDays(2)->format('Y-m-d');
    return "{$start} - {$end}";
});

So they key in the collection are correct but the data is being return group by weeks starting on Monday.