Today is the Jewish Day of Atonement, which in Hebrew is Yom Kippur (יום כיפור), with the stress on the second syllable of the second word. This is the stress that people in America have adopted ever since the so-called Yom Kippur War, which began when the Arabs launched a surprise attack in October 1973 on Israeli positions in the Israeli-occupied territories on Yom Kippur, the holiest day in Judaism.
Jews and Gentiles in America had theretofore conventionally pronounced Kippur with stress on the initial syllable, the stress in Yiddish, reflecting the Ashkenazic habit of retracting all stress in Hebrew disyllables onto the initial by comparison with the Sephardic pronunciation. For those in the know, this Yiddishized stress made the word sound the same as the English word kipper ‘a name given to the male salmon (or sea trout) during the spawning season’ (OED [“of uncertain etymology”]). The enormous publicity attending the Yom Kippur War gave pervasive currency to the Sephardic stress and all but obliterated the Ashkenazic one as far as American English was concerned, a situation lasting to this day.
American English, by contrast with British English, is prone to adopt in loan words, especially nomina propria, what is perceived to be “authentic,” hence the latter-day change in the second vowel of items such as Iraq and Iran from the traditional flat vowel to the current pervasive broad one.
Apropos of the Day of Atonement, when having nothing to atone for, one remembers Y-H-B’s father (who could read the Bible in Hebrew, Greek, and Latin) admonishing his son that the most important thing for a Jew to have is lev tov לב טוב ‘a good heart’.