Kiribati is among the first countries to enter the New Year because Kiritimati uses UTC+14.
Kiritimati, also called Christmas Island, is part of Kiribati and lies in the Line Islands. The islands use Line Islands Time, which is UTC+14, the most advanced standard time offset currently used in civil timekeeping. As a result, the local calendar date changes there before it changes in most of the world.
Kiribati moved the International Date Line eastward around its territory in 1994, helping place all of the country's islands on the same calendar day. Before that change, the country was divided by the date line.
“First country” can be an oversimplification because the exact order depends on which inhabited locations and territories are being compared. Kiritimati is the key place behind Kiribati's famous early New Year status.