Find max date less than slicer selected date

I feel like this should be really easy, but have spent many hours trying every possible solution I've seen, and none have worked.

I have a date table including Date, MonthEndDate, OneMonthPrior, PriorMonthEndDate.  I have a Transactions table with numerous fact fields, Date, MontheEndDate.

I have Date[Date] related to Transactions[Date], 1 to many, single direction

I have a slicer for Date[MonthEndDate]

I have a table with SWITCH measure for selected month which begins like:

VAR MonthEndDate = SELECTEDVALUE('Date'[MonthEndDate])
VAR TotalTrx = CALCULATE(SUM(Transactions[Total Trx]), FILTER('Transactions', 'Transactions'[MonthEndDate]=MonthEndDate))

I need a second column which is the month prior to the selected month.  I am struggling with the syntax... how do I calculate in the SWITCH measure for where the Transactions[MonthEndDate] = Date[PriorMonthEndDate] based on the selected Date[MonthEndDate}?  

Thanks!