You're likely thinking of the "Dark" versions of the seven Days in the original *The World Ends With You* game. These aren't planets, but rather alternate realities of the days' usual environments.

Here's how you unlock them:

1. Complete the regular version of each day. You need to finish the story and beat the Boss for the day.

2. Fulfill specific requirements. Each Dark Day has unique conditions that must be met before you can access it. This often involves:

* Collecting a certain number of pins.

* Completing specific sidequests.

* Visiting a particular location.

3. Speak to a specific character. After fulfilling the requirements, a character (usually a Shibuya resident) will approach you and offer you the chance to enter the Dark version of that day.

It's important to note that:

* Dark Days are incredibly challenging and are designed for players who have already completed the game once.

* Each Dark Day has its own special storyline and unique challenges.

* Dark Days have a significant impact on the game's ending.

If you're looking for a more in-depth guide on unlocking Dark Days, you can find helpful information on online wikis and walkthroughs dedicated to *The World Ends With You*. Good luck!