A date pun celebrating Komeiji Koishi from Touhou. The 14th of May is considered Koishi day because the numbers 5-14 can be read as go-i-shi [五一四] (ko-i-shi).
There is also an additional day dedicated to Koishi set to the 14th of June. The numbers 6-1-4 can be read as mu-i-shi which are the prefix for muishiki meaning "unconscious" or "unconsciousness".
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