seeing how shocked the mane 6 were when Cellestia announced it's her sister, i don't think anyone knew Luna and Nightmare moon were the same, it was quite an old story that most believed to just be a legend anyway
I thought the Summer Sun Celebration was simply Equestria's equivalent to the summer solstice. According to the MLP wiki, however, it was indeed a celebration of Celestia's victory over Nightmare Moon up until Luna returned.