Kiribati is often identified as the first country to enter a new calendar year.
The Pacific island nation spans a wide area and uses several time zones. Its Line Islands, including Kiritimati, observe UTC+14, the world's most advanced civil time zone. As a result, those islands reach midnight before most other inhabited places and are commonly described in media as the first country to welcome the New Year.
This answer needs a geographic qualification. Kiribati is not a single point, and its different island groups do not all enter the new year simultaneously. The “first country” label refers to the country containing the earliest-observing inhabited territory, especially the Line Islands. It does not mean every Kiribati island celebrates at the same instant.
Kiribati moved the International Date Line around its eastern islands in 1995 so its territory would share the same calendar date. Kiritimati is also known as Christmas Island, but it is not the Australian territory of Christmas Island in the Indian Ocean. Time-zone labels can change in political or administrative discussions, so the UTC+14 qualification is the key fact.