Oracle invalid identifier when using table alias

I have a query that creates two aliased tables using with statements:

WITH Rev AS
(SELECT *
   FROM FORECAST.REVENUE_SUMMARY
  WHERE FORECAST.REVENUE_SUMMARY.FEE_CD_ACT_SUM_ACCTG_DA >= to_date('10/01/2013', 'mm/dd/yyyy')
    AND FORECAST.REVENUE_SUMMARY.FEE_CD_ACT_SUM_ACCTG_DA < to_date('10/01/2014', 'mm/dd/yyyy')
),
Mon_pv as 
(select to_char(FEE_CD_ACT_SUM_ACCTG_DA, 'MON') as "Mon", 
        Fee_CD, 
        Fee_NM, 
        SUM(CASH_DAILY_CL) as "Collections" 
   from Rev 
 group by to_char(FEE_CD_ACT_SUM_ACCTG_DA, 'MON'), 
          Fee_CD, 
          Fee_NM
)
select distinct Mon 
  from Mon_pv
;

in the Mon_pv table "Mon" is just a derived month column. so in the final query all I want to do is select a list of distinct month names, but it gives me an error saying that "Mon is an invalid identifier." Ultimately I want to be able to use it in a pivot table to create columns out of the month names. But any query using "Mon" as a column throws an error. But when I use Select * from Mon_pv "Mon" appears as a column. Why does it throw an error when I mention the column name specifically in the final Select statement?