Walmart is open almost every day of the year during regular hours, beside from Christmas. Even then they are usually open for only a few hours less. They do not usually set holiday hours, so they should be open to pick up any last minute goods. To be certain, here is a link to the Walmart website where you can find a location near you and call to confirm: Walmart Store Locater.
Most other stores like Target, K-Mart, Best Buy, Macys, Sears, JC Penny, and your local grocery stores are going to be open.