Simple example of translating DAY_NUMBER to DAY_NAME using CASE WHEN:

SELECT 
      CASE 
          WHEN DAY_NUMBER = 1 THEN 'MONDAY'
          WHEN DAY_NUMBER = 2 THEN 'TUESDAY'
          WHEN DAY_NUMBER = 3 THEN 'WEDNESDAY'
          WHEN DAY_NUMBER = 4 THEN 'THURSDAY'
          WHEN DAY_NUMBER = 5 THEN 'FRIDAY'
          WHEN DAY_NUMBER = 6 THEN 'SATURDAY'
          ELSE 'SUNDAY'
      END AS DAY_NAME
FROM
      TBL_DAY_NUMBER;