MDX - Average for a Cube
Have you read the msdn documentation and looked at the examples?

https://msdn.microsoft.com/en-us/library/ms146067.aspx

Edit: Try this. 

WITH MEMBER Measures.[AVGSALE] AS 
AVG ( 
    DESCENDANTS(
          [DIMPROPERTYLENDER].[Hierarchy].[NAME],
          [DIMPROPERTY TIME].[TIMEDIM].[MONTH].ALLMEMBERS)        
          ), 
          [Measures].[VAL1]
    )

SELECT NON EMPTY Measures.[AVGSALE] ON 0, 
    NON EMPTY ([DIMPROPERTYLENDER].[Hierarchy].[NAME], [DIMPROPERTY TIME].[TIMEDIM].[MONTH]) ON 1

from [PropertyData2016]


PS: you might need to use [DIMPROPERTY TIME].[TIMEDIM].[MONTH].[MONTH] in your final select.