with member [Measures].[NazwaMiesiaca]
as '[Order Date].[Miesiac].CurrentMember.Name'
select  non empty {
head(order(
crossjoin({[Order Date].[Miesiac].children},
{[Order Date].[Dzien Miesiaca].children}),
[Measures].[Order Qty],
DESC
),1)
}
on columns
from [Adventure Works2014]
where [Order Date].[Rok].&[2013];