Very few things in the universe were equal, they lacked perfection. Although perfection wasn't as beautiful to Seven as it had once been, she still admired it. The fall solstice in the northern hemisphere was one of the few things that manged to achieve equal balance.
Seven found it one of the very limited things about Earth that was fascinating.
Day and night for one day completely equal, down to the very second. She rarely spent time on Earth, she wasn't welcome. For all the preaching humans did about acceptance it was very easy on Earth to see how much XBs disgusted them, the ever presence fear and resentment inn their eyes.
Once every few years she returned to for the fall solstice because she wanted to feel her connection to humanity, wanted to feel that connection to perfection.