Need help creating date delta columns

Hope you find a solution with lookupvalue() as andre suggested.

Regarding my solution, it won't work without having all dates in Date table..and the dates for visits are weekly ( my formula checks the previous day) 

From your first post you can actually do it in DAX or PowerQuery. 

1. My solution: "Add Index Column" in the reference query date you created load it and replace the delta measure second VAR with :

VAR previousvisits =CALCULATE (SUM ( Sheet1[Visits] );FILTER (ALL ( 'calendar' );'calendar'[Index]= MAX ( calendar'[Index] ) - 1))

If that not works check the links below..

For formatting DAX formulas (easier to look & understand) http://www.daxformatter.com/

2.A date dimention table will always be useful so - for creating a Date table take a look at this http://www.excelguru.ca/blog/2015/06/24/create-a-dynamic-calendar-table/

3.You can create the delta in PowerQuery by modifying a bit (change some functions)  this post http://www.powerquery.training/portfolio/time-intelligence-with-power-query/

4.Else if you need to do it in DAX, you will need a Date or Custom Date table. There is a really great article & site http://www.daxpatterns.com/time-patterns/

Hope that helps...