Question: Assuming I've created a pivot with Years and Months on the Rows panel and a Sales measure on the Columns panel.  How do I calculate the average sales per day for each month?

1. Create a Custom Measure by right-clicking the Measures node in the Data Browser.

2. Create the following formula: Avg([Days Level], [Sales]) by dropping the Days level from your Time dimension and your Sales measure onto the formula editor (above the main canvas), separating them by a comma and enclosing them within Avg().

3. Drop this new measure onto the Measures panel in the Widget Layout Editor (make sure you select the pivot first).

4. Click Update in the Widget Layout Editor. 

P.S.

Replacing Avg with Max and Min will calculate the maximum and minimum values per month respectively.

Replacing Avg with Count will calculate the number of unique days appearing in the data per month.

(Sum and DupCount are also available, but less relevant in this case)