Use of MySQL trigger to insert row's week number
You tagged MySQL, so here is what should work for a MySQL trigger on your Before Insert trigger.

 DELIMITER $$
 CREATE DEFINER=`USERNAMEHERE`@`%` TRIGGER `SCHEMANAMEHERE`.`TABLENAMEHERE_BEFORE_INSERT` AFTER INSERT 
 ON `TABLENAMEHERE` 
 FOR EACH ROW
 BEGIN
    SET NEW.WeekNumber = WEEKOFYEAR(NEW.InsertedDateTime);
 END$$
 DELIMITER ;