Get all nodes where date field matches month and year
You can use a range like condition:

$query = new EntityFieldQuery();
$query
  -&gt;entityCondition('entity_type', 'node')
  -&gt;entityCondition('bundle', 'horas')
  -&gt;propertyCondition('status', NODE_PUBLISHED)
  -&gt;fieldCondition('field_day', 'value', $myFieldDate . '-01', '&gt;=')
  -&gt;fieldCondition('field_day', 'value', $myFieldDate . '-31', '&lt;=');