How to generate moving Average of Last two days

Hi friend,  

These days I become little bit Lazy. The reason was my long leave from work. I visited my Hometown in Durga Puja. It was really very cheerful moment for me to meeting with parents and cousins. Please  don’t ask about my last 7 Days I was suffered with Eye-Flue. It isolated me becouse I was not  unable to use TV/Computer/Phone/Book, social media and SQL. After 6 days  now i become OK. Session of Diwali festival also comes up. So I should wake up and write some Diwali special Query. 

I started my SQL Server, opened SSMS and New Query Editor new problem …..

How to generate Running Average of Last two days?

Date	Day	Cost
08-Oct	Wed	[10]
09-Oct	Thurs	[15]
10-Oct	Fri	[20]
13-Oct	Mon	[30]
14-Oct	Tue	[40]

-----------------------------------------

create table #costCenter

(

dat  date,

days varchar(20),

cost  int

)

-----------------------------------------

insert into #costCenter(dat,days,cost)

select '10/08/2014','Wed',10 union

select '10/09/2014','Thurs',15 union

select '10/10/2014','Friday',20 union

select '10/13/2014','Mon',30  union

select '10/14/2014','Tues',40 

select * from #costCenter

------------------------------

SELECT     dat,days,cost ,avg(cost) OVER (

ORDER BY dat ROWS BETWEEN 1 PRECEDING AND CURRENT ROW

) AS AvgCOST

FROM   #costCenter

Result :