Is it necessary for everyone to see the splash page? Jay's method will only apply if they land on the site_start page. It won't affect people who follow a link to another page.
If you need everyone to see it, you could put a snippet at the top of your template with something like this:
<?php
/* SplashSnippet */
if (isset($_SESSION['splash_screen']) && $_SESSION['spash_screen'] == 'yes') {
return '';
} else {
$_SESSION['splash_screen'] = 'yes';
$modx->sendRedirect('url/of/splash/page');
}
Note that every user will see the splash page every time they visit the site, which could get pretty annoying.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html