Pleiades has a lot of symbolism that adds meaning to this story, so here are some tidbits:
Pleiades comes from the greek word plein, which means “to sail”.
It's associated with the beginning of the navigation season, when there’s not much ice in the sea, and it’s deemed safe to sail. This is why it’s often associated with themes of journey, navigating/choosing paths and (self) discovery.
It was believed that when Pleiades reaches its highest point in the sky at midnight, the veil separating the living from the dead is at its thinnest (And in this event story, there are heavy references of the novel Night on the Galactic Railroad, a story about 2 friends who boarded a magical train that travels between life and the afterlife.)
The cluster is famous for having 7 bright stars, but most people only see 6. The 7th star is said to be too “complicated and hard to understand” and has a wavering light that causes it’s brightness & visibility to vary, which is why it's been called as “The Lost Pleiad”. Note that this event's name is Lost Star＊Wavering Light, Pleiades Night.
Although no official statement has been released about it, Pleiades Night heavily references the famous short novel Night on the Galactic Railroad written by the Japanese author Kenji Miyazawa. The two protagonists share a relationship similar to Natsume and Tsumugi’s, and a few elements of their story can be symbolically found in the story. The train on the Star Spirit Festival stage could also be referencing the novel, as well as the one mentioned in its associated song Galaxy Destiny.
If you’re curious about it, you can read the novel for free here.