Kiribati is often among the first countries to celebrate New Year's Day because parts of it use UTC+14.
Kiribati is an island country spread across the central Pacific Ocean. Its Line Islands, including Kiritimati, use Line Islands Time, which is UTC+14, the world's most advanced civil time zone. As a result, midnight arrives there before it does in most other inhabited places.
The unusual arrangement is linked to Kiribati's decision in 1994 to move the International Date Line around its eastern islands. Before that change, different island groups in the country could be separated by a calendar day, making government administration and communication difficult.
The exact claim “first country” depends on whether one means the first inhabited location, the first territory, or the first part of a country to reach midnight. Kiribati is the standard country-level answer, while Kiritimati is the specific island most often named in New Year's coverage.