The Prime Minister says June 24 will be a public holiday in 2022 to celebrate Ngā Mata o te Ariki, more commonly known as Matariki, which traditionally heralds the start of a new year for Māori.
Labour promised during the election campaign to make Matariki a public holiday and was advised by Matariki Advisory Group when it should be celebrated across the country.
The Government has also asked the Matariki Advisory Group to advise on future dates, as the exact timing of Matariki shifts each year, but these are expected to always fall on a Monday or Friday.
"Matariki will be a distinctly New Zealand holiday; a time for reflection and celebration, and our first public holiday that recognises Te Ao Māori," Ardern said in her speech at Waitangi's upper marae on Thursday.
"It's great to have the date locked in for next year. This will be a day to acknowledge our nation's unique, shared identity, and the importance of tikanga Māori. It's going to be something very special, and something uniquely New Zealand," she said.
"It will also break up the lag between public holidays that currently exists between Queen's Birthday in early June and Labour Day in late October."
What is the meaning behind Matariki?
Matariki is a cluster of stars - known as Pleiades - that rises in mid-winter, traditionally marking the start of the Māori New Year.
Māori Crown Relations Minister Kelvin Davis says the reappearance of the Matariki stars in the sky each year marks the beginning of a fresh year for Māori, and in recent years its profile has grown and is celebrated across the country.
"Because the date of Matariki changes each year depending on the appearance of Pleiades in the sky, we have established a Matariki Advisory Group to provide advice on future dates of the public holiday, how it should be celebrated and to support the development of resources to educate the public on Matariki and the celebrations."
Davis said the advisory group will ensure that Mātauranga Māori - or Māori knowledge - is at the forefront of decision-making about the public holiday.
The group's chair Professor Rangiānehu Matamua says iwi mark the start of the Māori New Year in different ways around the country, so it's important this holiday acknowledges those regional differences.
Workplace Relations and Safety Minister Michael Wood said Matariki will help the tourism sector, as Kiwis plan more mid-winter getaways, and once the borders reopen.
"The Advisory Group will now work on recommended dates for the next 30 years to give businesses and communities' certainty."
Legislation will be introduced later this year to amend the Holidays Act and add in the additional public holiday.
Labour's announcement that it would introduce a new public holiday during the campaign was met with scepticism from the Opposition.
ACT leader David Seymour accused Ardern of being in "la la land".
"Does she know there's an economic crisis going on?" he asked, following Labour's announcement. "New Zealanders don't need a day off, they need Jacinda Ardern to take three years off.
"There are 70,000 more people on welfare, future generations face mountains of debt, businesses are struggling to survive, and Labour's answer is a new public holiday.
"If Labour wants Matariki to be a public holiday, it should abolish Labour Day so businesses aren't taking on more costs."
Employers and Manufacturers Association CEO Brett O'Riley said it would be seen as another cost and that the Government priority should be focused on fixing the Holidays Act.
"We need to see a simplified and streamlined process for calculating entitlements and creating efficiencies for business."