SQL Server 2017

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SET DATEFIRST 7; -- 7: Sunday has weekday 1
DECLARE @Test TABLE ([YEAR-WW] char(7)); 
INSERT INTO @Test VALUES 
  ('2020-53'), ('2021-01'), ('2021-02')
, ('2021-48')
, ('2021-53'), ('2022-01')
, ('2022-53'), ('2023-01')
;

SELECT [YEAR-WW]
, [FirstDayOfWeek] = CAST(DATEADD(day, 1-DATEPART(weekday, DATEADD(week, ABS(RIGHT([YEAR-WW],2))-1, LEFT([YEAR-WW],5)+'01-01')), DATEADD(week, ABS(RIGHT([YEAR-WW],2))-1, LEFT([YEAR-WW],5)+'01-01')) AS DATE)
, [LastDayOfWeek]  = CAST(DATEADD(day, 6, DATEADD(day, 1-DATEPART(weekday, DATEADD(week, ABS(RIGHT([YEAR-WW],2))-1, LEFT([YEAR-WW],5)+'01-01')), DATEADD(week, ABS(RIGHT([YEAR-WW],2))-1, LEFT([YEAR-WW],5)+'01-01'))) AS DATE)
FROM @Test
ORDER BY 1;


YEAR-WW
FirstDayOfWeek
LastDayOfWeek


2020-53
2020-12-27
2021-01-02

2021-01
2020-12-27
2021-01-02

2021-02
2021-01-03
2021-01-09

2021-48
2021-11-21
2021-11-27

2021-53
2021-12-26
2022-01-01

2022-01
2021-12-26
2022-01-01

2022-53
2022-12-25
2022-12-31

2023-01
2023-01-01
2023-01-07


CREATE FUNCTION dbo.GetDateFromYearWeek (
 @YearWeek VARCHAR(7) = '000101', -- default
 @WeekDay INT = 1, -- default
 @FirstWeekDayName VARCHAR(9) = 'mon' -- default
) RETURNS DATE
BEGIN
 
 IF @YearWeek = '000101'
   SET @YearWeek = CONCAT(DATEPART(year, GETDATE()), '-', DATEPART(week, GETDATE())); 
 
 IF @YearWeek NOT LIKE '[0-9][0-9][0-9][0-9]%[0-9-][0-9]'
   RETURN NULL;
 
 IF @WeekDay < 1 OR @WeekDay > 7
   RETURN NULL;
 
 DECLARE @FirstWeekDay INT = CHARINDEX(LOWER(LEFT(@FirstWeekDayName,3)), '   montuewedthufrisatsun')/3;
 IF @FirstWeekDay = 0 -- not found in string
   SET @FirstWeekDay = @@DATEFIRST;
 
 DECLARE @Year INT = TRY_CAST(LEFT(@YearWeek, 4) AS INT);
 DECLARE @Week INT = ABS(TRY_CAST(RIGHT(@YearWeek, 2) AS INT));
 
 DECLARE @Date DATE = TRY_CAST(CONCAT(@Year,'-01-01') AS DATE);
 SET @Date = DATEADD(week, @Week-1, @Date);
 
 DECLARE @DowDiff INT = (6-@FirstWeekday+@@DATEFIRST+DATEPART(weekday,@Date))%7;
 SET @Date = DATEADD(day, -@DowDiff, @Date);
 SET @Date = DATEADD(day, @WeekDay-1, @Date);

 RETURN @Date;
END;
CREATE FUNCTION dbo.GetDateFromYearWeekSundayFirst (
 @YearWeek VARCHAR(7),
 @WeekDay INT
) RETURNS DATE
BEGIN
  RETURN dbo.GetDateFromYearWeek(@YearWeek, @Weekday, 'sun');
END;
DECLARE @Test TABLE ([YEAR-WW] char(7)); 
INSERT INTO @Test VALUES 
  ('2020-53'), ('2021-01'), ('2021-02')
, ('2021-48')
, ('2021-53'), ('2022-01'), ('2022-02')
;

SELECT [YEAR-WW]
, [FirstOfWeek_Mon] = dbo.GetDateFromYearWeek([YEAR-WW], 1, 'mon')
, [LastOfWeek_Mon]  = dbo.GetDateFromYearWeek([YEAR-WW], 7, 'Monday')
, [FirstOfWeek_Sun] = dbo.GetDateFromYearWeek([YEAR-WW], 1, 'sun')
, [LastOfWeek_Sun]  = dbo.GetDateFromYearWeek([YEAR-WW], 7, 'SUNDAY')
FROM @Test
ORDER BY 1;


YEAR-WW
FirstOfWeek_Mon
LastOfWeek_Mon
FirstOfWeek_Sun
LastOfWeek_Sun


2020-53
2020-12-28
2021-01-03
2020-12-27
2021-01-02

2021-01
2020-12-28
2021-01-03
2020-12-27
2021-01-02

2021-02
2021-01-04
2021-01-10
2021-01-03
2021-01-09

2021-48
2021-11-22
2021-11-28
2021-11-21
2021-11-27

2021-53
2021-12-27
2022-01-02
2021-12-26
2022-01-01

2022-01
2021-12-27
2022-01-02
2021-12-26
2022-01-01

2022-02
2022-01-03
2022-01-09
2022-01-02
2022-01-08


create table datetest (SomeDate date);
with rcte as (
 select 1 as n, getdate() as dt
 union all
 select n+1, dateadd(day, 1, dt)
 from rcte
 where n < 10 
)
insert into datetest (SomeDate) 
select dt
from rcte;
10 rows affected

SET DATEFIRST 1;
declare @FirstWeekday INT = 1;
select SomeDate
, @FirstWeekday as wd1
, @@DATEFIRST df
, LEFT(DATENAME(weekday, SomeDate), 3) AS dayname
, DATEPART(weekday, SomeDate) as wd
, DowDiff = -(6-@FirstWeekday+@@DATEFIRST+DATEPART(weekday,SomeDate))%7 
from datetest


SomeDate
wd1
df
dayname
wd
DowDiff


2022-07-22
1
1
Fri
5
-4

2022-07-23
1
1
Sat
6
-5

2022-07-24
1
1
Sun
7
-6

2022-07-25
1
1
Mon
1
0

2022-07-26
1
1
Tue
2
-1

2022-07-27
1
1
Wed
3
-2

2022-07-28
1
1
Thu
4
-3

2022-07-29
1
1
Fri
5
-4

2022-07-30
1
1
Sat
6
-5

2022-07-31
1
1
Sun
7
-6

YEAR-WW	FirstDayOfWeek	LastDayOfWeek
2020-53	2020-12-27	2021-01-02
2021-01	2020-12-27	2021-01-02
2021-02	2021-01-03	2021-01-09
2021-48	2021-11-21	2021-11-27
2021-53	2021-12-26	2022-01-01
2022-01	2021-12-26	2022-01-01
2022-53	2022-12-25	2022-12-31
2023-01	2023-01-01	2023-01-07

YEAR-WW	FirstOfWeek_Mon	LastOfWeek_Mon	FirstOfWeek_Sun	LastOfWeek_Sun
2020-53	2020-12-28	2021-01-03	2020-12-27	2021-01-02
2021-01	2020-12-28	2021-01-03	2020-12-27	2021-01-02
2021-02	2021-01-04	2021-01-10	2021-01-03	2021-01-09
2021-48	2021-11-22	2021-11-28	2021-11-21	2021-11-27
2021-53	2021-12-27	2022-01-02	2021-12-26	2022-01-01
2022-01	2021-12-27	2022-01-02	2021-12-26	2022-01-01
2022-02	2022-01-03	2022-01-09	2022-01-02	2022-01-08

SomeDate	wd1	df	dayname	wd	DowDiff
2022-07-22	1	1	Fri	5	-4
2022-07-23	1	1	Sat	6	-5
2022-07-24	1	1	Sun	7	-6
2022-07-25	1	1	Mon	1	0
2022-07-26	1	1	Tue	2	-1
2022-07-27	1	1	Wed	3	-2
2022-07-28	1	1	Thu	4	-3
2022-07-29	1	1	Fri	5	-4
2022-07-30	1	1	Sat	6	-5
2022-07-31	1	1	Sun	7	-6