gistfile1.txt

$query = craft()->elements->buildElementsQuery();

//  First day of month included between agendaStartDate & agendaEndDate
$query->andWhere(
    '(content.field_agendaStartDate <= :firstDayOfMonth AND content.field_agendaEndDate >= :firstDayOfMonth) 
    OR
    (content.field_agendaStartDate <= :lastDayOfMonth AND content.field_agendaEndDate >= :lastDayOfMonth)
    OR 
    (content.field_agendaStartDate >= :firstDayOfMonth AND content.field_agendaEndDate <= :lastDayOfMonth)',
    [':firstDayOfMonth' => $firstDayOfMonth, ':lastDayOfMonth' => $lastDayOfMonth]
);

$entries = EntryModel::populateModels($query->queryAll());