Forecasting Sales

Dear Chaucer ,
You can use following measures , just instead of years you can use week as you want to forecast sales for 3 weeks ahead Calculating Sales
using DAX to give a Forecast by average for next year
Image has DAX for last 3 years and forecast for next year ,you can use it for week also 
if image is not clear 
VAR sales_last_year = Calculate([Total Sales],DATEADD(Date[Date] , -1 ,Year)) 
Such more 2 measures for calculating Sales for 3 years then 
use
return 
Divide all three variable (lastyear,2years back,3 years back) by 3 you will get forecast for next year 
remeber you need to replace years by week 


For more info you can check video which will help you a lot 
https://www.youtube.com/watch?v=vq3VOERJw7s

I would expect a kudos .
and if this helped getting close to your solution accept it as a solution

if any further help required please let me know , i think you can nail this problem with my recommended youtube video.

Regards ,

Sujit Thakur 

Thanks all for your help.

Using a combination of the links you guys added I knocked this up which works a treat. If i could select all as solution I would!