Kiribati moved west of the International Date Line in 1995 so its entire territory could share the same calendar day.
Kiribati is an island country spread across a vast area of the central Pacific. Before the change, its eastern islands could be nearly a day behind its western islands by local calendar date. The government shifted the date-line arrangement for the Line Islands, including Kiritimati, to remove that practical division.
The change made Kiribati one of the first places on Earth to enter a new calendar day. That is why Kiritimati is often featured in explanations of which inhabited locations celebrate New Year's first.
The date line is an international convention rather than a perfectly straight physical boundary. Kiribati's adjustment did not move the planet's rotation or create a new day; it changed the civil time-zone arrangement used by the country.