数字 (Sūji, numbers) have a special place in the Japanese language that's not all about 機能 (kinō, functionality), 数学 (sūgaku, math) or 測定 (sokutei, measurement).
In spite of their precise nature, numbers have always been allotted a lot of wiggle room, specifically when it comes to ダジャレ (dajare, puns) category.
For example, take the calendar. The 29th of every month has been deemed 肉の日 (Niku no Hi, Day of Meat), because of the 2(ni, two) and 9 (ku, nine). Get it? Nine can be read ku or kyū. Some people hold 肉の日 パーティー (Niku no Hi Pātii, Meat Day Parties) for a legitimate excuse to binge on barbecue and roasts. Sept. 1 is キウイの日 (Kiui no Hi, Day of the Kiwi) to match the Japanese pronunciation of the New Zealand fruit and 9-1 (kyūi). And so it goes.
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