営業日?

3営業日後

var_dump( nextBizDay( '2009-4-30', 3 ) );

<?php
define( 'DATE_CALC_FORMAT', '%Y-%m-%d' );
require_once 'Date/Calc.php';
require_once 'Date/Holidays.php';

function nextBizDay( $date, $after = 1 ) {
    list( $y, $m, $d ) = split( '-', $date );
    $dc = new Date_Calc( );
    
    if ( $after > 1 ) {
        $add = $after - 1;
        $res = $dc->addDays( $add, $d, $m, $y );
        list( $y, $m, $d ) = split( '-', $res );
    }
    
    do {
        $next = $dc->nextWeekday ( $d, $m, $y );
        $d++;
    }
    while ( isHoliday( $next ) );
    
    return $next;
}

function isHoliday( $date ) {
    list( $y, $m, $d ) = split( '-', $date );
    $dh = &Date_Holidays::factory( 'Japan', $y );
    return $dh->isHoliday( $date );
}