Talk to your DBA to fix territory. If it was set to e.g. India, then you wouldn't have to calculate anything:

SQL> alter session set nls_territory = 'India';

Session altered.

SQL> Select    to_char(sysdate, 'D') from dual;

T
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6

SQL>

In Oracle, the first day of a week is determined by NLS_TERRITORY parameter. So use a value of NLS_TERRITORY where Sunday is the first weekday, for example, AMERICA .

alter session set nls_territory=RUSSIA

select to_char(sysdate, 'D')
from dual

alter session set nls_territory=AMERICA

select to_char(sysdate, 'D')
from dual

fiddle

Option 1: NLS_TERRITORY session parameter

In Oracle, the day of the week depends on the NLS_TERRITORY session setting:

SELECT TO_CHAR(DATE '2022-12-11', 'D') AS day, -- a sunday
       (SELECT value FROM NLS_SESSION_PARAMETERS WHERE parameter = 'NLS_TERRITORY')
         AS territory
FROM   DUAL;

Outputs different values for different parameters. I.e., if you use:

ALTER SESSION SET NLS_TERRITORY='America';
ALTER SESSION SET NLS_TERRITORY='Germany';
ALTER SESSION SET NLS_TERRITORY='India';
ALTER SESSION SET NLS_TERRITORY='Oman';
ALTER SESSION SET NLS_TERRITORY='Bangladesh';

and run the query for each then the outputs are:
| DAY | TERRITORY |
| ------------ | ------------ |
| 1 | AMERICA |
| 7 | GERMANY |
| 1 | INDIA |
| 2 | OMAN |
| 3 | BANGLADESH |
So you could just pick the correct territory for your database, i.e.:

ALTER SESSION SET NLS_TERRITORY='India';

Option 2: Compare to the start of the ISO-week

Or if you want to be independent of the session settings then you can compare it to the start of the ISO-week (which is always midnight Monday) and offset by one day to change the comparison from Monday-Sunday to Sunday-Saturday:

SELECT TRUNC(SYSDATE + 1) - TRUNC(SYSDATE + 1, 'IW') + 1 AS day
FROM   DUAL;

Which, for today (Friday) outputs:
| DAY |
| ------------ |
| 6 |
fiddle

我找到了这个问题的答案，非常简单

select mod(to_char(sysdate,'D'),7)+1 as Weekday from Dual