How to create baseline sales

@Migscruz

I've tried to provide a solution based on what I think you're looking for, although I'm not quite sure what youmean when you say "average total sales every day and not by product."

However, to get started, I'll go through the process to calculate the average sales for the first 15 days of each month.

First the model (simple dataset):

In the calendar table, I added a YearMonth and Yearmonth Index column (the latter is a range in the YearMonth column that I'll use to filter the itching)

Now the measurements:

1) A simple sum of sales:

Sum of Sales = SUM('Sales Table'[Sales])

2) To calculate the sum of sales for the first 15 days of each month, I have created a cumulative sales measure for each month:

Cumulative Sum by month =
CALCULATE (
    [Sum of Sales],
    FILTER (
        ALL ( 'Calendar Table' ),
        'Calendar Table'[Date] <= MAX ( 'Calendar Table'[Date] )
            && 'Calendar Table'[YM Index] = SELECTEDVALUE ( 'Calendar Table'[YM Index] )
    )
)

3) Now we can isolate the sales value for the first 15 days of each month using:

Sales on day 15 =
CALCULATE (
    [Cumulative Sum by month],
    FILTER ( 'Calendar Table', DAY ( 'Calendar Table'[Date] ) = 15 )
)

What this table gives you:

To calculate baseline sales, use:

a) for sales including current month + 2 previous months:

Average Sales last 3 month =
IF (
    ISINSCOPE ( 'Calendar Table'[Month Name] ),
    AVERAGEX (
        FILTER (
            ALL ( 'Calendar Table' ),
            'Calendar Table'[YM Index]
                >= MAX ( 'Calendar Table'[YM Index] ) - 2
                && 'Calendar Table'[YM Index] <= MAX ( 'Calendar Table'[YM Index] )
        ),
        [Sales on day 15]
    )
)

b) for 3 previous months (excluding the current month):

Average Sales Previous 3 months =
IF (
    ISINSCOPE ( 'Calendar Table'[Month Name] ),
    AVERAGEX (
        FILTER (
            ALL ( 'Calendar Table' ),
            'Calendar Table'[YM Index]
                >= MAX ( 'Calendar Table'[YM Index] ) - 3
                && 'Calendar Table'[YM Index]
                    <= MAX ( 'Calendar Table'[YM Index] ) - 1
        ),
        [Sales on day 15]
    )
)

What this chart brings you:

If you can clarify the "product" point, we can polish these measures to meet your needs.