Value between Two Dates

Hi Creative_tree88 ,
To adapt the DAX formula to dynamically use the month of Sales Letter B Date (or default to the same month as Sales Letter A Date if Sales Letter B Date is blank), you can modify the calculation of _end to ensure it always picks up the correct month.

Updade DAX:

Value Between Two Dates Lookup = 
VAR _start = 'Finance Data'[Sales Letter A Date]
VAR _end =
    IF(
        ISBLANK('Finance Data'[Sales Letter B Date]),
        DATE(YEAR(_start), MONTH(_start), DAY(EOMONTH(_start, 0))), 
        DATE(YEAR('Finance Data'[Sales Letter B Date]), MONTH('Finance Data'[Sales Letter B Date]), DAY(EOMONTH('Finance Data'[Sales Letter B Date], 0)))
    )
VAR _key = 'Finance Data'[KEY]

VAR _codesInRange =
    FILTER(
        'Sales Data',
        'Sales Data'[Sales Date] >= _start &&
        'Sales Data'[Sales Date] <= _end &&
        'Sales Data'[KEY] = _key
    )

VAR _result =
    CONCATENATEX(
        _codesInRange,
        'Sales Data'[Code],
        ", "
    )

RETURN
IF(
    ISBLANK(_result),
    BLANK(),
    _result
)

if Sales Letter B Date is blank the end date is set to the last day of the month of Sales Letter A Date using EOMONTH(_start, 0).

If Sales Letter B Date is not blank it uses the last day of the month of Sales Letter B Date using EOMONTH('Finance Data'[Sales Letter B Date], 0).