Kiribati is widely associated with celebrating New Year's Eve first because its Line Islands use UTC+14, the world's most advanced civil time zone.
The key location is Kiritimati, also called Christmas Island, in Kiribati's Line Islands. When midnight arrives there, the date has advanced farther than in any other time zone used for ordinary civil timekeeping. This makes Kiritimati a frequent answer when people ask which inhabited place welcomes the new year first.
Kiribati moved the International Date Line around its eastern islands in 1994, bringing the country's widely separated islands onto the same calendar day. The change created a 24-hour difference between Kiribati's eastern and western island groups before later time-zone comparisons are considered.
The phrase “first country” is a convenient shorthand, not a claim that every part of Kiribati reaches midnight simultaneously. Kiribati spans multiple time zones, and the UTC+14 distinction specifically concerns the Line Islands.