Issue #3456417 by eugene bocharov, pablo_pukha, smustgrave, oily, andypost:...

   *   (optional) Whether to return the abbreviated name for that day.

   *   Defaults to TRUE.

   *

   * @return string|null

   * @return \Drupal\Core\StringTranslation\TranslatableMarkup|null

   *   The name of the day in the week for that date, or null if the $date has

   *   errors.

   */

    if (!$date->hasErrors()) {

      $dow = self::dayOfWeek($date);

      $days = $abbr ? self::weekDaysAbbr() : self::weekDays();

      return $days[$dow]->getUntranslatedString();

      return $days[$dow];

    }

    return NULL;

  }

use Drupal\Core\Datetime\DateHelper;

use Drupal\Core\DependencyInjection\ContainerBuilder;

use Drupal\Core\Language\Language;

use Drupal\Core\StringTranslation\TranslatableMarkup;

use Drupal\Tests\UnitTestCase;

/**

    $container = new ContainerBuilder();

    $config = ['system.date' => ['first_day' => 'Sunday']];

    $container->set('config.factory', $this->getConfigFactoryStub($config));

    $container->set('string_translation', $this->getStringTranslationStub());

    $this->languageManager = $this->createMock('\Drupal\Core\Language\LanguageManagerInterface');

    $language = new Language(['langcode' => 'en']);

    $this->assertNotNull(DateHelper::dayOfWeekName(FALSE));

    $this->assertNotNull(DateHelper::dayOfWeekName(''));

    // Ensure proper return value type.

    $this->assertInstanceOf(TranslatableMarkup::class, DateHelper::dayOfWeekName());

    // Pass nothing and expect to get NULL.

    $this->assertNull(DateHelper::dayOfWeekName(0));

    $this->assertNull(DateHelper::dayOfWeekName('0'));