Uppdaterar tabell med arbetsdag och kalenderdag

Här har du. Denna rekursiva CTE ger dig BD:erna för hela månaden:

declare @forwhichdate datetime
set @forwhichdate ='20170401'
;with bd as(
select 
DATEADD(DAY,
CASE
    (DATEPART(WEEKDAY, DATEADD(MONTH, DATEDIFF(MONTH, 0, @forwhichdate), 0)) + @@DATEFIRST - 1) % 7
    WHEN 6 THEN 2 
    WHEN 7 THEN 1
    ELSE 0
END,
DATEADD(MONTH, DATEDIFF(MONTH, 0, @forwhichdate), 0)
) as bd, 1 as n
UNION ALL
SELECT DATEADD(DAY,
CASE
    (DATEPART(WEEKDAY, bd.bd) + @@DATEFIRST - 1) % 7
    WHEN 5 THEN 3
    WHEN 6 THEN 2
    ELSE 1
END,
bd.bd
) as db, 
bd.n+1
from bd where month(bd.bd) = month(@forwhichdate)
)
select * from bd

Resultat:

bd                      n
----------------------- -----------
2017-04-03 00:00:00.000 1
2017-04-04 00:00:00.000 2
2017-04-05 00:00:00.000 3
2017-04-06 00:00:00.000 4
2017-04-07 00:00:00.000 5
2017-04-10 00:00:00.000 6
2017-04-11 00:00:00.000 7
2017-04-12 00:00:00.000 8
2017-04-13 00:00:00.000 9
2017-04-14 00:00:00.000 10
2017-04-17 00:00:00.000 11
2017-04-18 00:00:00.000 12
2017-04-19 00:00:00.000 13
2017-04-20 00:00:00.000 14
2017-04-21 00:00:00.000 15
2017-04-24 00:00:00.000 16
2017-04-25 00:00:00.000 17
2017-04-26 00:00:00.000 18
2017-04-27 00:00:00.000 19
2017-04-28 00:00:00.000 20
2017-05-01 00:00:00.000 21

(21 row(s) affected)

Men i verkligheten bör din fråga också söka efter semestern.