You cannot straightly query mongodb collections by date components like day or month. But its possible by using the special $where javascript expression
db.mydatabase.mycollection.find({$where : function() { return this.date.getMonth() == 11} })
or simply
db.mydatabase.mycollection.find({$where : 'return this.date.getMonth() == 11'})
(But i prefer the first one)
Check out the below shell commands to get the parts of date
>date = ISODate("2011-09-25T10:12:34Z")
> date.getYear()
111
> date.getMonth()
8
> date.getdate()
25
EDIT:
Use $where only if you have no other choice. It comes with the performance problems. Please check out the below comments by @kamaradclimber and @dcrosta. I will let this post open so the other folks get the facts about it.
and check out the link $where Clauses and Functions in Queries for more info