(Nice) Knee Socks Day, also called (Nice) Thighhighs Day or Nice Kneehighs Day, occurs on two dates: Nov 28th and Feb 8th. These are used to celebrate thighhighs and over-knee socks. Mostly this is equivalent with celebrating zettai ryouiki, especially in the 2D realm.
Thighhighs and over-knee socks are collectively called ニーハイ (nihai) in Japanese; these dates were thus chosen as word play: Nov 28th => 1 (ichi) - 1 (ichi) - 2 (ni) - 8 (ha(ch)i) = ii ni-hai; Feb 8th => 2 (ni) - 8 (ha(ch)i) = ni-hai
Note that this day isn't about the legwear in English/on Danbooru called "kneehighs", since the term ニーハイ is only used in above sense of just-above-knee length (aka over-kneehigh) to mid-thigh socks, as a kind of wasei-eigo. (Calf-/below-knee length socks are ハイソックス = high socks. サイハイソックス = thigh-high socks also exists, but only refers to socks that are almost leg length and thus have the appearance of tights (via the same logic as ニーハイソックス where the high part is used to mean "cover"-length rather than "reach"-length), so they aren't usually counted for zettai ryouiki).
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