On an alternative Earth live the people of Tcelonia.
One of their founding myths goes that, in the dawn of times, when they were under the menace of being attacked by an enemy population, for an entire month the night sky of the entire planet remained as bright as under the full moon, frustrating all the attempts of a surprise attack of the enemy who, at the end, gave up an left.
Since then, continues the myth, the favor of the gods has never left Tcelonia, which thrived and prospered.
What natural event can reasonably explain the characteristics of the phenomenon?
- Night sky as bright as when the full moon shines
- Duration of one month
- Spanning the entire dark side of the planet
- No severe and adverse effects on life
A single event is preferred, however I am also open to combination of more events leading to the same result. The less the events, the better.