Here is a small SQL showing that week 1 of the year starts with Jan. 1 and Sunday is day 1 of a week.
SELECT
a.d dDate,
TO_CHAR(a.d, 'Dy') cDay,
TO_NUMBER(TO_CHAR(a.d, 'd')) nDay,
TO_NUMBER(TO_CHAR(a.d, 'ww')) nWeek,
TO_NUMBER(TO_CHAR(a.d, 'ww')) +
CASE WHEN...