Yes, you need a Prime membership. Prime Big Deal Days is an exclusive sales event just like Prime Day before it and Prime Early Access Sale before that.
If you're not already a member, you'll need to sign up for Amazon Prime for $14.99 per month or $139 annually (plus tax) to qualify for discounts. For new users, a 30-day trial of Prime is the perfect way to take advantage since you're free to cancel your subscription after the event.
A Prime membership gets you fast and free shipping, discounts on groceries, access to video and music streaming services, games and streaming perks, and a lot more.