# app/code/Vendor/ModuleName/etc/frontend/events.xml

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="checkout_submit_before">
        <observer name="vendor_modulename_order_save_before" instance="VendorModuleNameObserverOrderObserver" />
    </event>
</config>

# app/code/Vendor/Name/Observer/OrderObserver.php
namespace VendorModuleNameObserver;

use MagentoFrameworkEventObserver;
use MagentoFrameworkEventObserverInterface;

class OrderObserver implements ObserverInterface
{

    public function __construct()
    {
    }

    /**
     * @param Observer $observer
     * @return void
     */
    public function execute(MagentoFrameworkEventObserver $observer)
    {
        $quote = $observer->getQuote();
        ...
        ...
        ...
    }
}