WordPress: Query for custom fields: Day, Month and Year

<?php
global $wp_query;

function filter_where($where = '') {

    if( !is_admin() ){

        if( isset($_GET['dia']) )
            $where.= " AND DAY(meta_value) = '". $_GET['dia']. "'";

        if( isset($_GET['mes']) )
            $where.= " AND MONTH(meta_value) = '". $_GET['mes']. "'";

        if( isset($_GET['ano']) )
            $where.= " AND YEAR(meta_value) = '". $_GET['ano']. "'";

    }
    return $where;
}

add_filter('posts_where', 'filter_where');

$args = array_merge( $wp_query->query, array( 'meta_key' => 'CUSTOM_FIELD_DATA', 'orderby' => 'meta_value', 'order' => 'desc' ) );
query_posts( $args );

//CUSTOM_FIELD_DATA: é o nome do seu custom field que contém a data. Lembrando que a data deve estar no formato YYYY-MM-DD
?>