Origin of the grammar of “mañana por X”-type phrases?

Question

I was wondering how phrases like mañana por la tarde and mañana por la mañana came about. Grammatically they seem really odd, as opposed to the simple "tomorrow afternoon" and "tomorrow morning," which make literal sense word for word.

How did Spanish come to phrase things this way?

Answer 1

por is a very versatile preposition. Depending on the context, it can mean by, through, at, in, on, for, because of... etc. In these constructions, the preposition acts similarly to the at or in in:

Tomorrow [in the] morning

or

Yesterday at dawn

For the equivalents of afternoon and morning a preposition isn't needed in some Romance languages (or English as you note):

• Fr Demain matin.
• It Domani mattina.
• Ro Mâine dimineață.

but in many other Romance languages one is used:

• Pt Amanhã de manhã.
• Gl Mañá pola mañá.
• Sp Mañana por la mañana.
• Ca Demà al matí.
• Co Dumane à dumani.

Given the split of languages here, and additionally knowing the Spanish phrase has had currency since the earliest Old Spanish documents¹ and that the Latin phrase crās māne lacked such a preposition, it seems this development occurred early in Iberian vulgar romance.

This may have evolved by analogy to how in Latin "tomorrow evening" was distinguished as a noun crastinus vesperi vs as an adverb cras ad vesperum, or by analogy to how per was used in other Latin time constructions² (e.g. per diem, per noctem, per Dionysia etc).

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Sources:

1. ^{...oy es día bueno e mejor será cras; por la mañana prieta todos armados seades...
   ...cras a la mañana pensemos de cavalgar...
   Poema de Mio Cid, 1140 - ("por la mañana", CORDE)}

2. ^{Nominal Apposition in Indo-European: Its Forms and Functions, and its Evolution in Latin-Romance, Brigitte L. M. Bauer (p.242-244)}

• ^{https://en.wikipedia.org/wiki/Vulgar_Latin_vocabulary}