.net - Linq Nested Grouping?

I've a large table of Items and I need to organize them by Category, then by Year and then by Month.Item has CategoryID and Dated properties.I got this far:

Dim Items = From Item In DB.Items _
Group By CategoryID = Item.CategoryID _
Into Categories = Group _
Order By CategoryID

But where I put the:

Group By Year = Year(Item.Dated)

and the

Group By Month = Month(Item.Dated)

The final result should be something like this:

For Each Category in Categories
For Each Year in Category.Years

[code].....