Average Days between 2 tables

You current measure calculates the average difference in days between the first sample and the first sale for each customer,
but it does not restrict the calculation to the specific month of the first sale.

MeasureName =

VAR SelectedMonthYear = MAX(Dates[MonthYear]) // Assuming you have a MonthYear column in your Dates table

VAR _TABLE =
    SUMMARIZE(
        Account,
        Account[Customer_Number],
        "@firstSale",
        CALCULATE(
            FIRSTDATE(SALES[SalesDate]),
            ALL(SALES),
            VALUES(SALES[Customer_Number])
        ),
        "@firstSample",
        CALCULATE(
            FIRSTDATE(SAMPLES[SampleDate]),
            ALL(SAMPLES),
            VALUES(SAMPLES[Customer_Number])
        )
    )

VAR FilteredTable = 
    FILTER(
        _TABLE,
        MONTH([@firstSale]) = MONTH(SelectedMonthYear) && YEAR([@firstSale]) = YEAR(SelectedMonthYear)
        && [@firstSample] < [@firstSale]
    )

VAR _result =
    IF(
        ISEMPTY(FilteredTable),
        BLANK(),
        AVERAGEX(
            FilteredTable,
            DATEDIFF([@firstSample], [@firstSale], DAY)
        )
    )

RETURN
    _result