Oberon is likely a character from the fairy realm. In this fanfiction, he is Harry Potter's mate. He might be based on the traditional Oberon from fairy lore, a powerful and somewhat mischievous fairy king who now forms a special relationship with Harry.
There could be a fanfiction where Oberon and Trinity team up for a mission on a desolate planet. Maybe they have to face a new and powerful enemy faction. Oberon's nature - based powers and Trinity's healing and energy - based abilities would make for an interesting combination. They could start off as just comrades in arms, but as the story progresses, a deeper bond of friendship might develop between them.
There might be many interpretations of 'Oberon Day'. If Oberon is from a certain mythology, perhaps it's a day dedicated to the character Oberon. Maybe it involves rituals, feasts, or special events related to the mythology he is part of. But again, this is all speculation without more context.
Some of the memorable characters in Oberon novels could be the brave protagonist, often on a quest to save their world or discover a great truth. There might also be the wise old mentor figure who guides the protagonist with their knowledge and experience. And don't forget the villain, whose evil plans drive much of the conflict in the story.
It is fiction. 'Shipmates' by Merle Collins is a fictional creation, crafted to entertain and tell a story that isn't directly based on real events or people.