The doujin seems to require a relatively detailed understanding of the "purple mirror" urban legend, which I can't say I have myself, and which isn't really explained in the story. It seems like there's a surprising amount of different versions and optional details referenced here, so I'll try to put up translator's notes once I've done a bit more digging around. It's going to be incomplete and weird until then.
EDIT: Added notes to some individual, plot-relevant references, but there's no good place to explain the whole legend. Summarized below for those curious, but I'd almost recommend reading the doujin first. Most of this was easily obtained on English Google, but some details were in Japanese Yahoo discussions or whatever.
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There was a very vain girl who spent all her time looking at herself in a mirror. Desperate to be beautiful, she developed anorexia, but no matter how sick she got, this particular hand mirror always showed her usual self. One day she decorated the mirror by painting the frames purple. However, the mirror suddenly showed her true self, and she broke it in disgust.
Later, on her 20th birthday, she was struck by a car and killed, muttering the words "Purple mirror, purple mirror..." while she died. The broken mirror was never found. However, other people around Japan soon started dying mysteriously on their 20th birthdays, and a shard of purple glass was always found nearby. Anyone who has the phrase "purple mirror" in their head on their 20th birthday is cursed to die.
Some versions only include the part about the phrase, without any particular backstory which seems to be a later addition. Some versions include phrases that'll protect you from the curse if you also remember them. Others involve different phrases but the same basic idea. Sometimes the curse only causes some sort of misfortune instead of death.