Check date field on form

Hello,

I’m making a website where you can register in activities. These activities (they are a WordPress custom post type) are only a few days a week. I want to make a form in WordPress to check the day I’m trying to register… For example, activity A can only be done on Mondays and Wednesdays. If I select Tuesday, Thursday, Friday, Saturday or Sunday form must give me error.

Anyone know how to do this in WordPress? Making a PHP script I see it, but I am new to WordPress and I have little idea how.

Thank you very much.
Regards