select d.date
   ,case
      when d.date < x.thisfirstmonday then datepart(year, x.prevjan4)
      when d.date >= x.nextfirstmonday then datepart(year, x.nextjan4)
      else datepart(year, x.thisjan4)
   end as isoyear
   ,case 
      when d.date < x.thisfirstmonday then 1 + datediff(day, x.prevfirstmonday, d.date) / 7
      when d.date >= x.nextfirstmonday then 1
      else 1 + datediff(day, x.thisfirstmonday, d.date) / 7
   end isoweeknumber
from (
      -- To demonstrate show week numbers for 14 days around today's date, 
      -- Januari 1st and December 31st.
      select dateadd(day, 7 - t.n, dt.date) as date
      from (
            select dateadd(day, datediff(day, 0, getdate()), 0) as date
            union select dateadd(year, datediff(year, 0, getdate()), 0)
            union select dateadd(day, -1, dateadd(year, 1 + datediff(year, 0, getdate()), 0))
         ) dt
         cross join (select 1 as n union all select 2 union all select 3
            union all select 4 union all select 5 union all select 6
            union all select 7 union all select 8 union all select 9
            union all select 10 union all select 11 union all select 12
            union all select 13 union all select 14
         ) t
   ) d
   cross apply (
      select max(case t.n when -1 then x1.jan4 else 0 end) as prevjan4
         ,max(case t.n when 0 then x1.jan4 else 0 end) as thisjan4
         ,max(case t.n when 1 then x1.jan4 else 0 end) as nextjan4
         ,max(case t.n when -1 then x2.jan4weekday else 0 end) as prevjan4weekday
         ,max(case t.n when 0 then x2.jan4weekday else 0 end) as thisjan4weekday
         ,max(case t.n when 1 then x2.jan4weekday else 0 end) as nextjan4weekday
         ,max(case t.n when -1 then x3.firstmonday else 0 end) as prevfirstmonday
         ,max(case t.n when 0 then x3.firstmonday else 0 end) as thisfirstmonday
         ,max(case t.n when 1 then x3.firstmonday else 0 end) as nextfirstmonday
      from ( 
            select -1 as n union all select 0 union all select 1
         ) t
         cross apply (
               select dateadd(year, t.n + datediff(year, 0, d.date), dateadd(day, 3, 0)) as jan4
            ) x1
         cross apply (
               select (-2 + datepart(dw, x1.jan4) + @@datefirst) % 7 + 1 as jan4weekday
            ) x2
         cross apply (
               select dateadd(day, 1 - x2.jan4weekday, x1.jan4) as firstmonday
            ) x3
   ) x
order by 1;

declare @DateOffset as smallint, @ThursdayOffset as smallint, @TargetDate as datetime,
 @CurrentThursday as datetime, @FirstThursday as datetime, @ISO_WeekNumber as tinyint

set @DateOffset = case when @@datefirst > 4 then @@datefirst - 8
 when @@datefirst > 1 then @@datefirst - 1 else 0 end
set @ThursdayOffset = case when @@datefirst < 5 then 5 - @@datefirst
 else 12 - @@datefirst end

set @TargetDate = getdate()

set @CurrentThursday = dateadd(day, @ThursdayOffset - 
 datepart(dw, @TargetDate + @DateOffset), @TargetDate + @DateOffset)

set @FirstThursday = dateadd(day, @ThursdayOffset -
 datepart(dw, cast('1/4/' + datename(year, @CurrentThursday) as datetime)
 + @DateOffset), cast('1/4/' + datename(year, @CurrentThursday) as datetime)
 + @DateOffset)

set @ISO_WeekNumber = datediff(week, @FirstThursday, @CurrentThursday) + 1

select
 TestDate = @TargetDate,
 CurrentThursday = @CurrentThursday,
 FirstThursday = @FirstThursday,
 ISO_WeekNumber = @ISO_WeekNumber,
 ISO_WeekYear = year(@CurrentThursday),

 ISO_WeekNumber_AllInOne = datediff(week, dateadd(day, case
 when @@datefirst < 5 then 5 - @@datefirst else 12 - @@datefirst end -
 datepart(dw, cast('1/4/' + datename(year, dateadd(day, case
 when @@datefirst < 5 then 5 - @@datefirst else 12 - @@datefirst end -
 datepart(dw, @TargetDate + case when @@datefirst > 4 then @@datefirst - 8
 when @@datefirst > 1 then @@datefirst - 1 else 0 end), @TargetDate + case
 when @@datefirst > 4 then @@datefirst - 8 when @@datefirst > 1 then
 @@datefirst - 1 else 0 end)) as datetime) + case when @@datefirst > 4 then
 @@datefirst - 8 when @@datefirst > 1 then @@datefirst - 1 else 0 end),
 cast('1/4/' + datename(year, dateadd(day, case when @@datefirst < 5 then
 5 - @@datefirst else 12 - @@datefirst end - datepart(dw, @TargetDate + case
 when @@datefirst > 4 then @@datefirst - 8 when @@datefirst > 1 then
 @@datefirst - 1 else 0 end), @TargetDate + case when @@datefirst > 4 then
 @@datefirst - 8 when @@datefirst > 1 then @@datefirst - 1 else 0 end))
 as datetime) + case when @@datefirst > 4 then @@datefirst - 8 when
 @@datefirst > 1 then @@datefirst - 1 else 0 end), dateadd(day, case when
 @@datefirst < 5 then 5 - @@datefirst else 12 - @@datefirst end -
 datepart(dw, @TargetDate + case when @@datefirst > 4 then @@datefirst - 8
 when @@datefirst > 1 then @@datefirst - 1 else 0 end), @TargetDate + case
 when @@datefirst > 4 then @@datefirst - 8 when @@datefirst > 1 then
 @@datefirst - 1 else 0 end)) + 1,

  ISO_WeekYear_AllInOne = year(dateadd(day, case when @@datefirst < 5 then
 5 - @@datefirst else 12 - @@datefirst end - datepart(dw, @TargetDate +
 case when @@datefirst > 4 then @@datefirst - 8 when @@datefirst > 1 then
 @@datefirst - 1 else 0 end), @TargetDate + case when @@datefirst > 4 then
 @@datefirst - 8 when @@datefirst > 1 then @@datefirst - 1 else 0 end))