1) "Somewhen" means "sometime (in the future)", combined with the meaning of "somewhere" sometimes. 2) "Somewhen comes the day" is an inversion, which goes like these sentences, "Here comes the bus", "There goes John". There plays the combining meaning, mentioned in 1). 3) Personally I have never, ever, heard any native speaker say this word "somewhen", probably because I am not lucky meeting time travellers from ancient times to our days :-) Could be found in artwork, poetry in Old English, I suppose. Frankly speaking, your question was once mine. I heard it from a Russian-native English-speaking friend, interestingly.

There is no such word as "somewhen". "Some day" or "somewhere" are words or phrases. Does that answer your question?

But i see that word on web. So i want to ask.

Somewhen is not a word. Please use a dictionary.

That sentence is not modern English. Notice how it uses the wrong tense and that 'comes' is before its subject. I've never seen 'somewhen' before and my spell-check says it's incorrect. I'd guess that it means "sometime" or "one day". Either way, it's not a word in modern English. I think that the sentence should be "Some day the day will come when war is over" in modern English.