>>56174438Generally it is a play on words when pronouncing certain numbers sounds like another word, for example Gardevoir day is pronounced 3/7 San - Nana/Shichi which is reminiscent of Sanaito (Sirknight), 16/7 sounds like Juu-roku - Nana/Shichi which I imagine that for a Japanese it sounds similar to Rukario.
It's silly, but in Japan they take these puns very seriously, to the extent that each generation has its group of secret words that only they know, making a combination of Hiragana, Katakana, Kanji and numbers whose phonetic combination only they understand.