Kiribati is widely recognized as the first country to enter a new calendar year because of its Line Islands time zone.
Kiribati's Line Islands use Line Islands Time, UTC+14, the world's farthest-ahead civil time zone. As a result, Kiritimati, also called Christmas Island, reaches midnight before most populated places on Earth. This makes the island a frequent focus of “first New Year” reports.
The situation changed in the 1990s after Kiribati shifted the date line around its eastern islands so that the country would share the same calendar date. Before the change, parts of Kiribati could be separated by a date difference of nearly a full day.
“First country” is a useful shorthand, but the exact wording matters: individual islands and time zones enter the new year at different moments. The claim refers especially to Kiribati's Line Islands, not every part of the country at one instant.