How can I translate it?

A simple solution:
after these lines in calendar_block.module file:

$calendar = (object) array(
    'year' => $year ? (int) $year : date('Y'),
    'month' => $month ? (int) $month : date('m'),
    'weekdays' => $weekdays,
  );

add the following:

$calendar->weekdays["su"]= "Sunday";
  $calendar->weekdays["mo"]= "Monday";
  $calendar->weekdays["tu"]= "Tuesday";
  $calendar->weekdays["we"]= "Wednesday";
  $calendar->weekdays["th"]= "Thursday";
  $calendar->weekdays["fr"]= "Friday";
  $calendar->weekdays["sa"]= "Saturday";