As Nintendo detailed on the Animal Crossing website, New Horizons will get seasonal items celebrating the Hinamatsuri festival, Pi Day, and St. Patrick's Day (also known as Shamrock Day in Animal Crossing) throughout March. As usual, you'll be able to order these items via the Seasonal tab in Nook Shopping. Shamrock Day clothing will also be on sale at the Able Sisters' shop next month. You can take a look at the new seasonal items below.
The aforementioned Super Mario items will be available to purchase in the game starting March 1, and they include Super Mushrooms, Thwomps, question blocks, and other Mushroom Kingdom staples, as will outfits based on Mario, Luigi, Wario, and Princess Peach. You'll also be able to purchase a set of warp pipes. Place these at two different points around your island and you'll be able to warp between them quickly.