public static function parseDays(string $annotation)
	{
		static $validValues = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun',];

		$days = NULL;
		$annotation = Strings::trim($annotation);
		if (Strings::length($annotation)) {
			$days = static::translateToDayNames($annotation);
			$days = static::expandDaysRange($days);
			foreach ($days as $day) {
				if (!in_array($day, $validValues)) {
					throw new InvalidParameterException(
						"Given day parameter '" . $day . "' must be one from " . implode(', ', $validValues) . "."
					);
				}
			}
			$days = array_values(array_intersect($validValues, $days));
		}

		return $days ?: NULL;
	}