Oracle 21c

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alter session set nls_territory=America;
with first_10_of_2020 as (
select date'2020-01-01'+level-1 dt from dual connect by level<=10
)
select 
   to_char(dt,'yyyy-mm-dd') dt
  ,to_char(dt,'day','nls_date_language=''English''') day_en
  ,trunc(dt)-trunc(dt,'iw')+1 as d_1
from first_10_of_2020;


DT
DAY_EN
D_1


2020-01-01
wednesday
3

2020-01-02
thursday
4

2020-01-03
friday
5

2020-01-04
saturday
6

2020-01-05
sunday
7

2020-01-06
monday
1

2020-01-07
tuesday
2

2020-01-08
wednesday
3

2020-01-09
thursday
4

2020-01-10
friday
5


alter session set nls_territory=France;
with first_10_of_2020 as (
select date'2020-01-01'+level-1 dt from dual connect by level<=10
)
select 
   to_char(dt,'yyyy-mm-dd') dt
  ,to_char(dt,'day','nls_date_language=''English''') day_en
  ,trunc(dt)-trunc(dt,'iw')+1 as d_1
from first_10_of_2020;


DT
DAY_EN
D_1


2020-01-01
wednesday
3

2020-01-02
thursday
4

2020-01-03
friday
5

2020-01-04
saturday
6

2020-01-05
sunday
7

2020-01-06
monday
1

2020-01-07
tuesday
2

2020-01-08
wednesday
3

2020-01-09
thursday
4

2020-01-10
friday
5

DT	DAY_EN	D_1
2020-01-01	wednesday	3
2020-01-02	thursday	4
2020-01-03	friday	5
2020-01-04	saturday	6
2020-01-05	sunday	7
2020-01-06	monday	1
2020-01-07	tuesday	2
2020-01-08	wednesday	3
2020-01-09	thursday	4
2020-01-10	friday	5