Slicer to compare data with previous months

I am Not sure the intention of the slicer, but you can try like this.

 

a) Create a new parameter with numeric range

         1 to 31 with increment 1

 

Change the slicer as range

 

 

b) Optional: To test, you can do this measure

Selected Days = MIN('Days input'[Days input]) & " - " & MAX ('Days input'[Days input])

Create a card using this measure "Selected Days"

See if the slicer and the measure works correctly! it should match based on your selection.

 

c) Assuming

   c. 1. your Fact/transaction table and date table are created and relationships are set properly.

   c. 2. 'Dim Date' table has day column. We use this column to filter.

   c. 3.  Create new measure like this:

Count Rows - Filter Days Input = 
var _begin = Min('Days input'[Days input])
var _end = Max('Days input'[Days input])

return CALCULATE(countrows('Fact Sales'), -- Replace countrows with sum, count or ... as per your needs
          Filter('Dim Date', 'Dim Date'[Day] >= _begin && 'Dim Date'[Day] <= _end)
    )
    

 

Adjust the formula to your needs. i.e., countrows('Fact Sales') to meet your needs like sum, count ... and, change the 'Dim Date' table with your date table.

 

c.4. Use this measure in your matrix 

 

Hope this help!