If the screen presented in the lightbox when the page loads is critical enough that it needs to happen every time any user visits the homepage, then perhaps consider incorporating it onto the page itself as the first thing users see when they land on 'home'.

The forced interaction of requiring a user to make an additional click on the 'x' to move on to anywhere else on your site is frustrating. (Considering the banner is also ever-present this seems redundant)

Edit: Lightbox also showing up on your About page - it's a bit much at this point.