Actual bye day vs Budget by month Analysis

Hello guys!

I'm new with Power BI, and I'm trying to achieve a really common scenario.

I have experience with another tool that create relationships in a fairly easy way, but I'm not sure how to do this.  I have two spreadsheets, one with all account movements by day and the budget table, by month.

I had to pivot my budget table, like this:

to 

And created a first of month column on my "Account movements table"

With this, I can no longer create a relationship between the "IT Concept" column because I don't have the many to many option, so I created another IT Concept table. But my problem is that I cannot link the StartofMonth column, to my Date column on the pivot table, as it is a many to many relationship.

Do I need to create a fourth table with dates only to achieve this?

I'll really appreciate any help. Best regards!