Go to sleep and enter this code: DA-6382-1459-4417
So Nintendo's new dream island is finally open for business in Animal Crossing: New Horizons, and it's pretty easy to get to.
Although there is nothing extraordinary about the game limits, it allows you to complete the Mario mission quickly and easily. Here's a quick overview.
If you haven't played New Horizons in a while (rolls hand again), start it up and head to your house. Go to bed and select the prompt to fall asleep and not just lay down (too real).
After that, you will enter the dream realm. Select "Search by Dream Address" from the prompt. Then enter the code [DA-6382-1459-4417] for "Ninten" Island. Note that "DA-" is automatically part of the string, you just need to type in the numbers.
From there, you'll wake up on the island of Nintendo and you should be ready to complete the Mario mission "Visit the Official Nintendo Island in the Animal Crossing: New Horizons game." Just to be sure, I'd go back to sleep on the bed, go home, then save your game.
Note that you will "receive credit for the mission within seven days of the end of the event." Given that it ends on March 24 at 7:59 a.m. PT, it's going to take some time before you actually see the fruits of your labor. Also note that you have until then to do so!