WordPress Lightbox Demo Page - Auto Open Once
The following lightbox popup will automatically open on page load and it will only open once in 24 hours.
To test the effect again, clear cookies of your web browser and reopen the page.
Clicking the link below will manually open the lightbox:
YouTube Lightbox Auto Open Once on Page Load
For how to create the demo, please view the tutorial: How to automatically open a lightbox popup on page load.