SQL函数因特定输入而超时（while循环）

我想获得给定月份的第一个工作日。我生成一个假期表并将其与常规工作日检查结合使用，以查找一个月的第一个工作日。似乎有效。但是，当我使用第一个月作为输入时，例如(2020, 1)功能超时。这样的每一年，每隔一个月都有效。

CREATE FUNCTION FirstBusinessDay(@year INT, @month INT) RETURNS DATETIME
AS BEGIN
    DECLARE @calendar TABLE (Name varchar(80), Date DATETIME)
    INSERT INTO @calendar SELECT * FROM dbo.HolidayTable(@year)
    DECLARE @d DATETIME = DATEFROMPARTS(@year, @month, 1)
    DECLARE @isHoliday bit
    SELECT @isHoliday = CASE WHEN Name IS NULL THEN 0 ELSE 1 END
    FROM @calendar WHERE Date = @d
    WHILE DATEPART(DW, @d+@@DATEFIRST-1) > 5 or @isHoliday = 'true'
    BEGIN
        SET @d = DATEADD(DAY, 1, @d)
        SELECT @isHoliday = CASE WHEN Name IS NULL THEN 0 ELSE 1 END
        FROM @calendar WHERE Date = @d
    END
    RETURN @d
END;
GO

用法示例

select dbo.FirstBusinessDay(2020, 1) as 'First Workday';  -- timeout
select dbo.FirstBusinessDay(2020, 2) as 'First Workday';  -- works 
select dbo.FirstBusinessDay(2020, 3) as 'First Workday';  -- works
select dbo.FirstBusinessDay(2021, 7) as 'First Workday';  -- works
select dbo.FirstBusinessDay(2020, 12) as 'First Workday'; -- works
select dbo.FirstBusinessDay(2021, 1) as 'First Workday';  -- timeout
select dbo.FirstBusinessDay(2022, 1) as 'First Workday';  -- timeout

如您所见，有一种模式，每次使用第一个月都会超时。谁能发现这个问题？