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Adding "の" substantivizes the phrase. Only use it if you really mean it as a name!
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Revised by Gemini
Grammar nuance: Description vs. Label

Why is "皮が赤いのリンゴ" weird?

Without の: You are describing the apple's appearance. "Look, a red apple." (Natural)

With の: You are turning "Red Skin" into a Title. "Look, the 'Red Skin' model apple." (Unnatural)

Adding "の" substantivizes the phrase. Only use it if you really mean it as a name!
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明日ハレの日ケの昨日：Tomorrow will be Special; Yesterday was not
Too common words to indicate a Yoimiya. I‘d render it into (the night) after normal days before the festival. Too specific, maybe. Then, expose “the night” after normal days before “the special”.

神々が恋した幻想郷：The Gensokyo the Gods Loved
It should be either - the gods missed or - where the gods loved someone.