Create a column with [date-field].[month] to fetch the months. Now add the column to the slicer to select the month.
Create a measure to fetch an average month by dropping sales in the card and converting to average to find average sales.
Create a column to check the days and return day with the highest sale
Column = IF(Orders[Sales]=MAX(Orders[Sales]),(Orders[Order Date].[Date]),DATE(1900,1,1)).
I have given DATE(1900,1,1) as default date to all dates except highest sale day.
Now use the measure to calculate the max(column) to fetch the day with the highest sales.
Hope it helps.