The actual Long Night did not begin until the army of the dead controlled by the Others from the Lands of Always Winter moved past the Wall and invaded the lands of Westeros for the first time in 8000 years.
Winter Arrives
The devasting winter that led to The Long Night began in the turn of a new century, 300 AC. The War of the Five Kings was in full swing, by this point, and only half way through. However, while all the kingdoms of Westeros waged war on one another, a great enemy emerged to the north Beyond the Wall.
The Others and their army of wights descended first upon the Free Folk. The Free Folk, or wildlings, then turned south themselves, following their king, Mance Rayder, to attack the Wall and Night's Watch.
Jon the White Wolf was Lord Commander of the Night's Watch in 300 AC., according to the order's records. He defeated the Wildling armies, but instead of slaughter, he took them in if they accepted peace. The long-term enemies turned into allies and began defending the realm of men from the Others, together.
From 300 AC. to somewhere between 304-310 AC. the Wall was able to withstand the attacks. It is unclear how they were able to survive the siege of the Others, however, some believe magic to be involved. Some stories of The Long Night told today, speak of Danaerys the Mother of Dragons arriving with her forces to help slow the assault of the dead. However, nothing has been able to be proven about this time in the North or beyond because there are nearly no records from this time to survive.
In 303 AC. Stannis Baratheon and his Northmen allies marched south to fight for his claim, as opposed to going north to help the Night's Watch. During the Siege of the Crossing, Stannis died in the assault, ending his war for the throne. It is known the Northmen returned to the North following the defeat, however, what they did after remains unclear. For the next few decades, the North was not heard from by the South.
Most historians believe they most likely found their way to the Wall, once the threat of the Others and wights became too real.
The North is Invaded
While the exact timing of the Others' invasion of the North is unclear, it is agreed upon that it occurred between 304 AC. and 310 AC. following a failure at the Wall leading to the army of wights ravaging the southern lands.
What exactly occurred at the Wall to allow such an invasion is unknown, although, large cracks and damage remain to the East where it is believed the Others breached the defenses. This is also the event used to signify the beginning of The Long Night by historians.
The Northmen fought the wights and Others with only the remaining survivors from the Night's Watch under Jon the White Wolf's command, the Wildlings, and possibly Daenerys the Mother of Dragons.
This invasion is widely accepted as the primary reason for such a long silence from the kingdom to the South.
Ice Meets Fire
Following the invasion of the North, the army of wights slowly began to make their way south. It was not until some time later that the wights truly made it past the neck in large quantities. Prior to this, sights of the creatures were scattered but consistent, leading many lords to become wary of the possibilities.
Once the army of the dead made it to the South, the first major conflict was the Sack of Seagard. Stories told from The Long Night say Lord Patrek Mallister, thereafter also known as the "Savior of Seagard", saw the army of wights approaching the Riverlands port town, he rang the Booming Tower to warn the town. This is seen as the first major conflict of what later became known as the War for the Dawn.
King Aegon Blackfyre called all the forces of the realm of men to arms, including the independent kingdoms of Vale and Iron Islands. Slowly the army of the living pushed the wights and Others back to the North and eventually beyond the Wall once more.
With the Others beyond the Wall again in 340 AC., winter finally began to come to an end and The Long Night was officially over. It took several more years for the spring to arrive, and to this day, wights still haunt the kingdom of the North, but for the most part, Westeros has begun to return to a state of normalcy in the 60 years following The Long Night.