If you need this for the current date then you can use either of the following:

SYSTEM(ADMIN)=> select to_char(now(),'Month');
  TO_CHAR
-----------
September

(1 row)

SYSTEM(ADMIN)=> select to_char(current_date,'Month');
  TO_CHAR
-----------
September
(1 row)

If you need it for a date value stored in a column of a table, simply change now() or current_date to the column name and add a "from table tablename" to the SQL.