In the Netherlands, there are two official Christmas holidays: December 25 and 26. Especially on December 26, more and more shops are open, as the rules for closure tend to liberalize. Especially in Amsterdam centre, you'll find many shops open.
Most attractions are closed (but eg. the Rijksmuseum is open 365 days a year).
Restaurants, esp. the better one, can be fully booked on these dates, so be early if you can.
Christmas in Holland is mostly celebrated as a family get-together, with lots of food, next to a decorated christmas tree.
The giving of presents is becoming more and more popular, although giving presents at Sinterklaas (December 5) remains popular too - especially when there are still small children.
Of course, churches are busy with Christmas too. Many churches have services and mass at Christmas eve. These events tend to be very popular, and often it is impossible to get in if yoo're not early.
In Amsterdam you'll see decorated shop windows, and many large illuminated christmas trees in town. The biggest one is traditionally on Dam Square.
A new Christmas 'tradition' in Amsterdam is the Amsterdam Light Festival, which is held every year in November, December and January. This consists of a walking tour along several works of art that are made with light. Also, there is a canal tour along these works of art.
Christmas tree on Nieuwmarkt
Christmas tree Dam Square