Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
SELECT SYSDATE, TRUNC(SYSDATE, 'IW') FROM DUAL

SYSDATE
TRUNC(SYSDATE,'IW')

2024-02-27 21:31:34
2024-02-26 00:00:00

SELECT SYSDATE, NEXT_DAY(SYSDATE - 7, 'MONDAY') FROM DUAL

SYSDATE
NEXT_DAY(SYSDATE-7,'MONDAY')

2024-02-27 21:31:34
2024-02-26 21:31:34

ALTER SESSION SET NLS_DATE_LANGUAGE = 'French';
SELECT SYSDATE, NEXT_DAY(SYSDATE - 7, 'MONDAY') FROM DUAL
ORA-01846: not a valid day of the week