Oracle 21c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD';
CREATE OR REPLACE FUNCTION dow(P_DAY DATE) RETURN VARCHAR2
IS
BEGIN
    RETURN(TO_CHAR(P_DAY,'DAY'));
END;
/
 with starting_date (datum) as
      (select add_months(trunc(sysdate, 'yyyy'), 12) from dual)
    select          add_months(datum, level - 1)  first_day,
           dow(first_day),
          last_day(add_months(datum, level - 1)) as last_day,
            dow(last_day)
    from starting_date
    connect by level <= 12;
ORA-00904: "LAST_DAY": invalid identifier

with starting_date (datum) as (
  select add_months(trunc(sysdate, 'yyyy'), 12) from dual
)
select
  add_months(datum, level - 1) as first_day,
  dow(add_months(datum, level - 1)) as first_dow,
  last_day(add_months(datum, level - 1)) as last_day,
  dow(last_day(add_months(datum, level - 1))) as last_dow
from starting_date
connect by level <= 12;


FIRST_DAY
FIRST_DOW
LAST_DAY
LAST_DOW


2023-01-01
SUNDAY
2023-01-31
TUESDAY

2023-02-01
WEDNESDAY
2023-02-28
TUESDAY

2023-03-01
WEDNESDAY
2023-03-31
FRIDAY

2023-04-01
SATURDAY
2023-04-30
SUNDAY

2023-05-01
MONDAY
2023-05-31
WEDNESDAY

2023-06-01
THURSDAY
2023-06-30
FRIDAY

2023-07-01
SATURDAY
2023-07-31
MONDAY

2023-08-01
TUESDAY
2023-08-31
THURSDAY

2023-09-01
FRIDAY
2023-09-30
SATURDAY

2023-10-01
SUNDAY
2023-10-31
TUESDAY

2023-11-01
WEDNESDAY
2023-11-30
THURSDAY

2023-12-01
FRIDAY
2023-12-31
SUNDAY

FIRST_DAY	FIRST_DOW	LAST_DAY	LAST_DOW
2023-01-01	SUNDAY	2023-01-31	TUESDAY
2023-02-01	WEDNESDAY	2023-02-28	TUESDAY
2023-03-01	WEDNESDAY	2023-03-31	FRIDAY
2023-04-01	SATURDAY	2023-04-30	SUNDAY
2023-05-01	MONDAY	2023-05-31	WEDNESDAY
2023-06-01	THURSDAY	2023-06-30	FRIDAY
2023-07-01	SATURDAY	2023-07-31	MONDAY
2023-08-01	TUESDAY	2023-08-31	THURSDAY
2023-09-01	FRIDAY	2023-09-30	SATURDAY
2023-10-01	SUNDAY	2023-10-31	TUESDAY
2023-11-01	WEDNESDAY	2023-11-30	THURSDAY
2023-12-01	FRIDAY	2023-12-31	SUNDAY