出典:Wiktionary
From Middle English dayes, dawes, from 古期英語 dagas, from Proto-Germanic *dagōs, *dagōz, plural of *dagaz, equivalent to day + -s (複数形 ending).
days pl (複数形 only)
From Middle English daies, from 古期英語 dæġes (“by day”), from Proto-Germanic *dagas, *dagis, genitive of *dagaz, equivalent to day + -s (adverbial ending).
「days」は名詞「day」の複数形です