EDIT: See the other answers for better solutions
The original newbie approaches that I offered were (opt1):
$qb->where("e.fecha > '" . $monday->format('Y-m-d') . "'");
$qb->andWhere("e.fecha < '" . $sunday->format('Y-m-d') . "'");
And (opt2):
$qb->add('where', "e.fecha between '2012-01-01' and '2012-10-10'");
That was quick and easy and got the original poster going immediately.
Hence the accepted answer.
As per comments, it is the wrong answer, but it's an easy mistake to make, so I'm leaving it here as a "what not to do!"