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 (c. 550).

ἡ ἡμέρα τοῦ κυρίου - the day of the Lord - appears in the major 4th and 5th century Codices: Sinaiticus (4th c.), Alexandrinus (5th c.), Vaticanus (4th c.), Ephraemi (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,"¹ 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, 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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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 (c. 550).

ἡ ἡμέρα τοῦ κυρίου - the day of the Lord - appears in the major 4th and 5th century Codices: Sinaiticus (4th c.), Alexandrinus (5th c.), Vaticanus (4th c.), Ephraemi (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,"¹ 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, in the 16th century. I don't believe it is known exactly where Erasmus acquired his texts, though they most likely came from sources more or less associated with the Orthodox Patriarchate of Constantinople.

^{1. Preface}