It would be better to take the first letter of day from the translated version

in date_api.module (line 175) we have:

function date_week_days_abbr($required = FALSE, $refresh = TRUE, $length = 3) {
  if ($refresh || empty($weekdays)) {
    $weekdays = array();
    foreach (date_week_days_untranslated($refresh) as $key => $day) {
      $weekdays[$key] = t(substr($day, 0, $length));
    }
  }
  $none = array('' => '');
  return !$required ? $none + $weekdays : $weekdays;
}

With this method, I have to work twice to translate both, complete days and first letter of the days. It is not a big deal, but the fact that some days start with the same letter makes it imposible to translate the first letter of the day correct.

I suggest changing the method to use the translated days and get the first letter already translated:

function date_week_days_abbr($required = FALSE, $refresh = TRUE, $length = 3) {
  if ($refresh || empty($weekdays)) {
    $weekdays = array();
    foreach (date_week_days($refresh) as $key => $day) {
      $weekdays[$key] = substr($day, 0, $length);
    }
  }
  $none = array('' => '');
  return !$required ? $none + $weekdays : $weekdays;
}