It seems like a bad design to me to require ThumbnailDisplayGroup and ThumbnailMonthGroup where one is an HTML container and the other is just some kind of virtual container mostly used for date comparison.

If we just do monthly groups (and not month range groups) then you'll obviously need just one class.  But even for grouped months, I think you can do it with just one class. I think the extra layer of abstraction just makes things more complicated.