Kiribati is widely recognized as the first country to enter the New Year because of its time zone.
The key location is Kiritimati, also called Christmas Island, in Kiribati’s Line Islands. The islands use UTC+14, the world’s furthest-forward civil time zone, so midnight arrives there before it does in most other populated places.
Kiribati shifted the International Date Line around its eastern islands in 1995 so the country’s territory would share the same calendar date. Before that change, the islands on opposite sides of the line could be separated by a day despite belonging to one nation.
“First country” is a practical New Year description rather than a claim that every island in Kiribati reaches midnight simultaneously. Kiribati spans several time zones, so Kiritimati is the relevant place for the earliest national celebrations.