Restrict use of coupons on weekends

I'm using woocommerce booking and I need generate coupon restriction for the bookings.

All my bookings have a duration of one hour and the customers can use coupons to reduce the cost of the booking but I need restrict the use of this coupons based in the day when the reservation will be used, not the day when the booking is generated.

For example, I have a coupon and I made my reservation on Mon for a room in Sat, In that case, the coupon must be not good. But if I try to book a room any day of the week for next Monday for example, the coupon must be good.

In this case, I need restrict the use for weekends.

So far my code is:

add_filter( 'woocommerce_coupon_is_valid', 'coupon_week_days_check', 10, 2);
 function coupon_week_days_check( $valid, $coupon ) {

     // Set HERE your coupon slug   <===  <===  <===  <===  <===  <===  <===  <===  <===  <===
     $coupon_code_wd = 'couponyes';
     // Set HERE your defined invalid days (others: 'Mon', 'Tue', 'Wed', 'Thu', 'Fri' )  <===  <===
     $invalid_days = array( 'Sat', 'Sun');

     $now_day = date ( 'D' ); // Now day in short format

     // WooCommerce version compatibility
     if ( version_compare( WC_VERSION, '3.0', '<' ) ) {
         $coupon_code = strtolower($coupon->code); // Older than 3.0
     } else {
         $coupon_code = strtolower($coupon->get_code()); // 3.0+
     }

     // When 'xyz' is set and if is not a week day we remove coupon and we display a notice
     if( $coupon_code_wd == $coupon_code && in_array($now_day, $invalid_days) ){
         // if not a week day
         $valid = false;
     }
     return $valid;
 }

Any idea?

You need a slight different code targeting the booking "start date" when it's submitted on add to cart action. So try this instead:

// Utility function to check coupon code 'abcxxr'
function check_coupon_code( $coupon ){
    // Set HERE your coupon slug
    $coupon_code_wd = 'abcxxr';

    $coupon_code = strtolower($coupon->get_code()); // WC 3.0+
    $coupon_code_wd = strtolower($coupon_code_wd);
    $found = false;

    // Loop through cart items to get the chosen day and check it
    foreach( WC()->cart->get_cart() as $cart_item ){
        if( $coupon_code_wd == $coupon_code && isset( $cart_item['booking']['_date'] ) ){
            $the_day = date('D' , strtotime($cart_item['booking']['_date']));
            if( in_array( $the_day, array('Sat', 'Sun') ) ){
                $found = true;
            }
        }
    }
    return $found;
}

// Coupon validity checking
add_filter( 'woocommerce_coupon_is_valid', 'coupon_week_days_check', 10, 2);
function coupon_week_days_check( $valid, $coupon ) {
    if( check_coupon_code( $coupon ) ){
        $valid = false;
    }
    return $valid;
}

// Coupon validity checking error message
add_filter('woocommerce_coupon_error', 'coupon_week_days_error_message', 10, 3);
function coupon_week_days_error_message( $err, $err_code, $coupon ) {
    if( intval($err_code) === WC_COUPON::E_WC_COUPON_INVALID_FILTERED && check_coupon_code( $coupon ) ) {
        $err = __( "This coupon $coupon_code_wd is valid for week days only…", "woocommerce" );
    }
    return $err;
}

Code goes in function.php file of your active child theme (or active theme). Tested and works.