ἡ ἡμέρα τοῦ Χριστοῦ - *the day of (the) Christ* - is found in the majority of Greek manuscripts as well as in one version of the [Codex Claromontanus][1] (c. 550).

ἡ ἡμέρα τοῦ κυρίου - *the day of the Lord* - appears in the major 4th and 5th century Codices:  [Sinaiticus][2] (4th c.), [Alexandrinus][3] (5th c.), [Vaticanus][4] (4th c.), [Ephraemi][5] (5th c.).

The NIV editors state that they base their translation on an "eclectic" Greek text, "based on the latest editions of the Nestle-Aland/United Bible Societies Greek New Testament,"<sup>1</sup>  which in turn opted for the latter reading of the verse over the former.

The King James Version is based on a collection of Greek texts compiled by the Dutch Roman Catholic priest, [Desirderius Erasmus][6], in the 16th century.  I don't believe it is known exactly where Erasmus acquired his texts, though it is accepted they came from sources more or less associated with the Orthodox Patriarch of Constantinople.



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<sup>1.</sup> <sup>Preface</sup><br/>


  [1]: https://en.wikipedia.org/wiki/Codex_Claromontanus
  [2]: http://www.codexsinaiticus.org/en/
  [3]: https://en.wikipedia.org/wiki/Codex_Alexandrinus
  [4]: https://en.wikipedia.org/wiki/Codex_Vaticanus
  [5]: https://en.wikipedia.org/wiki/Codex_Ephraemi_Rescriptus
  [6]: https://en.wikipedia.org/wiki/Textus_Receptus