How to group a list or library by month if you have a field containing Dates?

Simple, create a calculated field in your list or library that returns a string.

Use this code: =TEXT(Date,"yyyy - ")&TEXT(Date,"mm")&TEXT(Date," (mmmm")&TEXT(Date," yyyy)")

Now group by this field and that's it!

You can find everything on functions that you can use in calculated fields on: http://office.microsoft.com/en-us/sharepointtechnology/HA011609471033.aspx