Kiribati is famous for celebrating New Year’s first because part of the country uses the UTC+14 time zone.
Kiribati is an island republic in the central Pacific Ocean. Its territory stretches across a huge area, so the country historically had date-line complications. In 1995, Kiribati moved the International Date Line around its eastern islands by changing the relationship among its time zones, allowing its inhabited islands to share the same calendar date.
Kiritimati, also called Christmas Island, lies in Kiribati’s Line Islands and uses Line Islands Time, which is UTC+14. That is the world’s furthest-forward civil time zone, so midnight arrives there before it does in locations using lower UTC offsets.
“First country to celebrate” is a shorthand, not a claim that every person in Kiribati reaches midnight at exactly the same moment: the country spans time zones. The specific geographic distinction usually concerns the Line Islands, especially Kiritimati, rather than the entire nation simultaneously.