Is there English counterpart to Japanese proverb, 一期一会 , meaning "Cherish once -in -a-lifetime encounter"?
Oishi-san, as a former advertising professional you might enjoy knowing that the saying that possibly comes closest to this is possibly an old ad slogan, apparently from the mid-'60s:
You never get a second chance to make a first impression.
See this thread and a search for the expression leads to many others.
Advertising, of course, is well known for borrowing ideas, so this may not be original with the copywriter who penned it. But I think the saying fits.
In the far more general sense, there is carpe diem ("seize the day"), "you only go around once" (or "I shall pass through this world but once"), "make hay while the sun shines", and of course, "YOLO".
You cannot step twice into the same river is in proverbial use in Czech, and I've thought of it as such, but in fact it appears to be a quote from Heraclitus.
The observation would be that seemingly ordinary, everyday events of life are in fact unique.
The Yale Dictionary of Modern Proverbs (2012) does have this proverb about recognizing a good thing too late:
You never know what you have till it's gone (you've lost it).
1952 William Goyen, Ghost and Flesh (New York: Random House) 82: "'Well,,' I said, 'I guess that's the way life is, you don't know what you have till you don't have it any longer, till you've lost it, till it's too late." 1954 A.C. Friend, "The Proverbs of Serlo of Wilton," Medieval Studies 16: 212 (given as a loose translation ofr equivalent of Seneca's "nihil magis placent quam quod amissum est"): "You don't know what you have until you have lost it.
A generation of Canadian and U.S. young people became aware of this expression in the early 1970s through the song "Big Yellow Taxi," by Joni Mitchell, which has as its chorus:
Don't it always seem to go
That you don't know what you've got till it's gone
They paved paradise and put up a parking lot
In all of these English-language manifestations, the proverb seems to be saying, "We should have cherished the thing we had (whether a personal relationship, a beautiful environment, or a deeply meaningful moment); but our natural tendency is to take what we have for granted until we no longer have it." It is certainly a more pessimistic way of looking at a possible once-in-a-lifetime encounter than the Japanese proverb encourages people to adopt.