'Come' as a subordinate conjunction

Question

Is 'come' a subordinating conjunction in the following sentences?

'Come next February, he should have completed his degree.'

'He'll be wishing he'd arrived at Oslo come the winter.'

Answer 1

[Come next February], he should have completed his degree.

I would classify this "come" as a preposition.

Historically, this is a subjunctive clausal construction with "come" a plain form verb and "next February" its subject; synchronically, however, its function and internal structure are like that of a PP (compare "by next Friday"), and it is plausible to suggest that "come" has been reanalysed as a preposition.

The same applies to your other example.

Answer 2

There is an assumption here that there is a valid answer to what is actually an interesting question. The Cambridge English Dictionary gives an example of this type of expression: "

informal Come Monday morning [= when it is Monday morning) you'll regret staying up all night)].

There is no attempt to parse here. It is treated purely as idiomatic.

Merriam Webster offers a different definition:

to come to pass : take place —used in the subjunctive with inverted subject and verb to express the particular time or occasion

By way of example it offers: Come spring the days will be longer.

So Webster thinks of come in this context is a subjunctive. It does not offer a further explanation.

Collins English Dictionary offers a third perspective. Under its explanation numbered 10, we find that it can be a preposition.

You can use come before a date, time, or event to mean when that date, time, or event arrives. For example, you can say come the spring to mean 'when the spring arrives'. Come the election on the 20th of May, we will have to decide. He's going to be up there again come Sunday.

So according to Collins, come in this usage serves as a preposition.

Etymonline offers a useful perspective on the matter.

elementary intransitive verb of motion, Old English cuman "to move with the purpose of reaching, or so as to reach, some point; to arrive by movement or progression;" also "move into view, appear, become perceptible;

Come Spring clearly refers to the arrival and in that sense is not the same is in Spring. It has the advantage of brevity, by comparison with at the start of Spring.

But why, in that case are there no other such participial or subjunctive use, such as 'end winter'? Well, language development is not necessary logical or consistent. At some time come could have been used in this way, was understood and imitated. The process of language development does not involve anyone thinking about existing grammar or semantics. It is sufficient for such a locution to be immediately understood. It can be called a colloquialism or, as Cambridge, informal. Duke Ellington used the locution as the title of his wonderful composition Come Sunday.