Convert date to day name e.g. Mon, Tue, Wed

This is what happens when you store your dates and times in a non-standard format. Working with them become problematic.

$datetime = DateTime::createFromFormat('YmdHi', '201308131830');
echo $datetime->format('D');

See it in action

echo date('D', strtotime($date));
echo date('l', strtotime($date));

Result

Tue
Tuesday

For other language use day of week to recognize day name

For example for Persian use below code

$dayName = getDayName(date('w', strtotime('2019-11-14')));

    function getDayName($dayOfWeek) {

        switch ($dayOfWeek){
            case 6:
                return 'شنبه';
            case 0:
                return 'یک شنبه';
            case 1:
                return 'دو شنبه';
            case 2:
                return 'سه شنبه';
            case 3:
                return 'چهار شنبه';
            case 4:
                return 'پنج شنبه';
            case 5:
                return 'جمعه';
            default:
                return '';
        }

    }

More info : https://www.php.net/manual/en/function.date.php

Your code works for me

$date = '15-12-2016';
$nameOfDay = date('D', strtotime($date));
echo $nameOfDay;

Use l (lowercase L) instead of D, if you prefer the full textual representation of the name.