How to Get 21st date of the any month in sql server

SELECT DATEADD(d,
case when(datepart(dd,DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE()),0)))=31) then -11
when(datepart(dd,DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE()),0)))=30) then -10
when(datepart(dd,DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE()),0)))=29) then -9
else -8 end,
DATEADD(mm, DATEDIFF(m,0,GETDATE()),0))


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