The Unscheduled Night

The Unscheduled Night was a major event in which Nightmare Moon returned to Epona after her one-thousand year banishment within the Moon.

Prelude
What took place before the raising of the land bridge is unknown.

21st June
Without warning, a wave travels across the surface of the Moon, wiping away the Mare in the Moon as all the loose dust on the surface collects into one location. Nightmare Moon's land bridge forms from the collected dust and begins to reach off the surface of the Moon towards Epona. At the moment the end of it impacts the ground, day turns to night and Nightmare Moon's invasion begins.

Consequences
Nightmare Moon is turned back into Luna, severely injuring her in the process - the Moon is unable to be moved from its current position in the sky because of this.

Twilight Sparkle, Applejack, Rainbow Dash, Rarity, Pinkie Pie and Fluttershy collectively become the Bearers of Harmony.

The surface of the Moon is wiped clean, completely erasing the iconic Mare in the Moon.

Millions of tonnes of lunar material is deposited upon the surface of Epona - widespread damage to buildings in Hammerton and the complete destruction of Everfree Castle.

Other Notes

 * Twilight Sparkle theorizes that Nightmare Moon could have returned earlier, but chose to wait for the one-thousand year anniversary of her banishment for the sake of making a grander return.