The seven days run from the 28th day of the last lunar month to the 5th day of the new year. That is five holiday days plus two weekend days moved.
Companies must give at least five days and pick one of three patterns: one day before Tết and four after, two before and three after, or three before and two after. They must tell their staff at least 30 days ahead.
What shops and restaurants do is their own choice, and no official page says. Many close for some of those days; ask the ones you plan to visit.