Does anyone know the origin of why mano is feminine and día is masculine? I understand the explanation for most of the other contrary gender words.
mano y dia
This article talks about why certain words are exceptions to the gender rules. However, he says that some are simply just exceptions with no explanation. " La mano" and "el dia" are listed in that "just because" category.
I can't speak to día, but the reason mano is feminine is because manus, in Latin, is feminine, hence so is mano.