Hello,
Refer link :
SQL SERVER – Find Last Day of Any Month – Current Previous Next
In Short
SELECT DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE())-1,0))
SELECT DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE()),0))
SELECT DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE())+1,0))
SELECT DATEADD(s,-1,DATEADD(mm, DATEDIFF(m,0,GETDATE())+2,0))