Extension hook for add and edit form before saving to db?

Here’s my scenario. I have a “Featured on Landing Page?” custom field drop down (landing-featured) with “Yes” and “No” as options. For each weblog I only want one entry to have the ability to be featured on a landing page. Currently, two or more entries can be featured on the landing page which isn’t what I want. This can be solved if I update all entries for landing-featured to NULL, an empty string, or “No” and then let EE process the form as usual. I also want this to happen to affect specified weblogs and specifically when I add or edit an entry. I want to know what extension hook is appropriate for what I want. Any ideas?

Thanks ahead of time :D.

Moved by Moderator to Extensions: Technical Assistance