WITH SET [Current Day] AS
   {
     Tail (
       NonEmpty ( [Date].[Calendar].[Date].Members, [Measures].[Reseller Sales Amount] ),
       1
     ).Item ( 0 )
   }

Reading the MDX from the inside out, I first use the NonEmpty function to identify only days in our date dimension that have a Reseller Sales Amount. So as long as we process the cube nightly, the last date in the set returned should be the latest day. Then I use the Tail function to return the last member in the set, which will be the last day.

WITH SET [Current Day] AS
   {
     Tail (
       NonEmpty ( [Date].[Calendar].[Date].Members, [Measures].[Reseller Sales Amount] ),
       1
     ).item(0)
   }

MEMBER [Measures].[Current Day Sales Amount] AS

   Aggregate ( [Current Day], [Measures].[Reseller Sales Amount] )

MEMBER [Measures].[Current Day Order Qty] AS

   Aggregate ( [Current Day], [Measures].[Reseller Order Quantity] )

SELECT { [Measures].[Current Day Sales Amount], [Measures].[Current Day Order Qty] } ON 0

FROM [Adventure Works]

Here I created a couple calculated measures: Current Day Sales Amount and Current Day Order Qty. Here is the result from the query: