SQL Server 2017

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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;

col	StartOfWeek_SundayFirst	StartOfWeek_MondayFirst	EndOfWeek_SundayFirst	EndOfWeek_MondayFirst
202201	2021-12-26	2021-12-27	2022-01-01	2022-01-02
202202	2022-01-02	2022-01-03	2022-01-08	2022-01-09