It is a disadvantage that in English the verb without ending has so many uses: infinitive, 5 forms of present tense, present subjunctive, imperative, and if the past participle has the same stem form as the first as in come/came/come it can even be a past participle.
This disadvantage has as effect that in some cases it simply is unclear which form it is.
The expression "come+time" is a when-indication. Longman's DCE has in come no. 30 the examples: come July/next year/the next day. There is even a sentence:
- Come spring, you'll have plenty of colour in the garden.
In my view "come" is no present subjunctive in the sense of wishing. That would make not much sense.
"come" is a past participle, and a very shortened form of "when July/spring
has come". Probably derived from an absolute with-construction, eg With Troy distroyed Aeneas set off for Italy. In such with-constructions "with" can be dropped so that we have noun + participle. And we may guess that in a very frequent when-indiction the participle was placed in front:
- "With spring come" shortened to "spring come" and then "come spring".
A pity that Longman's DCE gives only examples and the sense, but doesn't say which verb form it is.
It is possible to compare the English expression to French: Une fois Décembre venu, nous aurons de la neige. Word-for-word translation: Once December (has) come, we'll have snow. The French might also say: Venu Décembre (Come December).
It is interesting to see how dictionaries explain this "come". Oald: preposition. Oxford COD: subjunctive. MacMillan: preposition. Merriam-Webster:subjunctive. No dictionary has a feeling that a subjunctive makes no sense.
In answer to @siride, comment 5. - You are right, the expression "come+time"seems the only expression with that structure. It is a very frequent when-indication and I don't find it astonishing that there is only one such formula. Same case with ago. We only have one such pseudo-preposition after a noun, derived from a verb form agone. I find it very improbable that "when spring comes" should be transformed into "when spring come (without -s), and then in "come spring". Such a series of transformations has no logic and I doubt that speakers would have the idea for such transformations.