Conditionally setting a Variable , sql

I'm trying to do this:
--If the Day parameter is the current day, then set @EndDate to be yesterday instead.
--If the Day parameter is in the future, set @EndDate to yesterday.

I've tried a few different approaches including two down below. I'm fairly new to programming so odds are I'm missing something fairly simple. Basically, I am trying to set @EndDate conditionally, depending on what @Day is set to be.

Declare @Day datetime    
Set @Day = '09/2/17 12:50'  
SET @Day = DATEADD(dd, DATEDIFF(dd, 0, @Day), 0)

Declare @Enddate Datetime  
Set @Enddate = Case @Day  
            When @Day < GETDATE() Then GETDATE() - 1
            End  

--Set @Enddate = @Day  
    --Where @Day < GETDATE()  
--Set @Enddate = GETDATE()-1    
    --Where @Day >= GETDATE()

,Thanks