In Icelandic folklore, northern Lights are often viewed as a sign of good luck. Witnessing them was considered an omen of good fortune, especially for those about to embark on a journey or a significant life event.

What Are the Northern Lights?

The phenomenon known as the Northern Lights occurs when energized particles from the sun collide with Earth's upper atmosphere at staggering speeds of up to 45 million mph (72 million kph).

In ancient times, some believed the Northern Lights to be the restless spirits of their fallen foes, seeking vengeance and foretelling of impending doom and conflict.

Today, this awe-inspiring display of lights is revered as the ultimate spectacle in skywatching, often referred to as the Holy Grail of celestial phenomena.

Ref: https://www.space.com/15139-northern-lights-auroras-earth-facts-sdcmp.html

Ideal Time for Aurora Viewing:

Between September and April marks the prime period for witnessing the northern lights, when the sky darkens sufficiently to unveil the mesmerizing aurora. In far northern regions, the phenomenon of the midnight sun, characterized by 24 hours of daylight during the summer, prevails. Typically, the peak activity occurs from 9 p.m. to 3 a.m., as detailed by the Geophysical Institute of the University of Alaska Fairbanks.

Nordic Folklore: The Vikings believed the Northern Lights illuminating the sky were the reflections of the Valkyries' armour as they led the warriors to Odin.

Valkyrie are female warriors who would choose who may die in battle and who may live to fight another day.

Odin was the great magician among the gods. 

Other Nordic legends claim the aurora was the breath of brave soldiers who died in combat.

Odin, the esteemed chief deity and ruler of Asgard, commanded reverence from all Vikings. It was their conviction that during each earthly battle, Odin would select the warriors destined to join him in Valhalla, his celestial abode.

The Valkyries, formidable female warriors mounted on steeds and clad in armor with spears and shields, bore the responsibility of escorting Odin's chosen fighters to Valhalla. As the Northern Lights adorned the heavens, the Vikings perceived them as the radiant reflections of the Valkyries' gleaming armor, guiding the warriors to Odin's realm.

In additional Nordic lore, the aurora was described as the exhalation of valiant soldiers fallen in combat. Alternately, in tales portraying the aurora as the 'Bifrost Bridge,' a luminous, undulating archway guided deceased warriors to their ultimate sanctuary in Valhalla.

Danger in the lights?

For the Sámi, the indigenous Finno-Ugric people of northern Scandinavia, the Northern Lights did not narrate tales of heroism and valor; instead, they commanded both fear and reverence. The manifestation of the aurora was perceived as an ill omen.

Regarded as the spirits of the departed, the Sámi culture cautioned against speaking of the Northern Lights. Engaging in gestures like waving, whistling, or singing beneath them was deemed perilous, as it could draw the attention of the lights.

According to their beliefs, attracting the lights' notice might result in being lifted into the sky or even facing decapitation by their luminous touch! Presently, many Sámi opt to remain indoors when the Northern Lights grace the sky, erring on the side of caution. 

Ref: https://www.hurtigruten.com/en/inspiration/northern-lights/myths-legends

According to Cree beliefs, the Aurora bore profound symbolism as a component of life's cycle. They perceived the lights as celestial manifestations of departed souls, existing in the sky yet distant from their earthly kin. The Cree held the conviction that these luminous displays represented the spirits of their departed loved ones endeavoring to establish communication with those they had bid farewell to on Earth.

https://en.wikipedia.org/wiki/Cree

The Cree are a North American Indigenous people primarily residing in Canada, where they constitute one of the country's largest First Nations. Most of the Cree population in Canada is found north and west of Lake Superior, specifically in Ontario, Manitoba, Saskatchewan, Alberta, and the Northwest Territories. Historically, the Cree in the United States lived westward from Lake Superior. Today, they are predominantly located in Montana.