Use
Sample.objects.filter(date__range=["2011-01-01", "2011-01-31"])
Or if you are just trying to filter month wise:
Sample.objects.filter(date__year="2011",
date__month="01")
Edit
As Bernhard Vallant said, if you want a queryset which excludes the specified range ends
you should consider his solution, which utilizes gt/lt (greater-than/less-than).