From Halopedia, the Halo wiki
Ten Twenty was a documentary series on Halo Waypoint, consisting of one video and three articles, which related events leading up to and occurring on October 20, 2552. Released on November 1, 2009 in promotion for Halo 3: ODST, it ties together the events of Halo: First Strike, Halo 2, and Halo 3. The documentary series was originally featured in the "Universe" section of Halo Waypoint, but was re-released with updated Halo 2: Anniversary graphics to mark the addition of Halo 3: ODST to Halo: The Master Chief Collection and can now be found on the "Learn" section of the Halo Channel.
Halo Waypoint description: THE EVENTS ON AND AROUND OCTOBER 20, 2552. A look at the events leading up to the Covenant's invasion of Earth....
Halo Channel Description: When Regret arrived near the planet on October 20, 2552, the prophet was so intoxicated by religious fervor and the untapped potential of the Forerunner machine that he ignored the strength of Earth's defenses and sped toward the planet's surface.
Narrator: October 20th, 2552: Ten Twenty, the date the Covenant first arrived on Earth. Although this date remains important, the seeds of the invasion of Earth were planted well before it. Not long prior to the discovery of Halo, the Prophet of Truth learned of a portal which could lead the Covenant to the Ark, the key to their "Great Journey". This gateway had remained hidden for a hundred-thousand years just below the surface of Earth. Soon after, Truth began secretly amassing a fleet of over five-hundred ships, all preparing to attack humanity's homeworld. Reports indicate that a detachment of Spartans uncovered this plot. On September 13th, these Spartans lead a UNSC strike-force to destroy this fleet, temporarily halting Truth's plans for Earth. But another Hierarch, the Prophet of Regret, had far different plans.... Ignorant to humanity's presence on Earth, Regret did not hesitate to bring his own fleet to the planet, hoping to quickly locate the ancient gateway. However, when he arrived on October 20th the Prophet soon found himself outnumbered and outgunned and was eventually forced to flee, destroying most of New Mombassa in the process. Hours later, Truth was able to salvage this botched invasion, sending his infantry to the Earth's surface to continue where Regret had left off. Ten Twenty, and the days which followed it, marked a time of both fear and hope for the UNSC. While the unearthing of the Ark's gateway would be dark and foreboding, its destination offered an end to the Covenant and the key to humanity's survival.
Halo Waypoint description: The first invasion efforts and the Spartans' early victory.
Although Halo 3: ODST begins on October 20, 2552, its story really starts much earlier. Some time before the Covenant's occupation of Earth, the Prophet of Truth managed to discover the location of humanity's home world—a distant, backwater planet known as Earth. Seeing this as an opportunity to obliterate the last stronghold of his enemy, he went quickly to work.
Truth swiftly commissioned a massive fleet and privately summoned over five hundred warships to a command-and-control station called Unyielding Hierophant. When UNSC forces uncovered this plot during the events of Halo: First Strike, they initiated a covert assault of the station. Led by the Master Chief, a strike team of SPARTAN-II super-soldiers infiltrated Unyielding Hierophant and destroyed it, along with the vast majority of ships in its charge.
Halo Waypoint description: The Prophet of Regret's inadequate invasion force collapses.
Shortly after the station was destroyed, the Prophet of Regret also discovered something significant about the planet Earth—it hid an ancient Forerunner artifact of great implication. Buried below its surface was an ancient machine capable of generating a portal to a remote facility known as the Ark—the place where the Covenant could finally initiate their Great Journey. Yet Regret remained oblivious to the fact that the planet was humanity's home.
Anxious to exploit the artifact's power, Regret led a meager detachment of fifteen ships to Earth. After Truth learned of Regret's brash and foolhardy move, he quickly sent an enormous support and excavation fleet to Earth.
When Regret arrived near the planet on October 20, 2552, the Prophet was so intoxicated by religious fervor and the untapped potential of the Forerunner machine, he ignored the strength of Earth's defenses and sped toward the planet's surface. At the coastal city of New Mombasa, he ferried down troops in search of this machine but was soon forced to retreat when UNSC forces converged on his position.
While Regret was unable to locate the Ark's gateway portal, he managed to divine the location of a new Forerunner ringworld—Delta Halo—to which he would escape, ultimately meeting his own end during the events of Halo 2.
Halo Waypoint description: Truth's plan and the Covenant's occupation of Earth.
The city of New Mombasa was nearly destroyed by Regret's venture into Slipspace, and the Covenant forces which remained above the planet were now without a leader. Although the Elite commanders continued to engage the human forces in space, the Prophet of Truth had moved his own plans forward—machinations which would forever change the Covenant.
Well before even that day, Truth had begun to doubt the reliability and loyalty of the Elites, who were, at that time, the heart and head of the Covenant military. Though Regret's ineffective invasion of the human home world was problematic, Truth would use it to his advantage. Not only was the support and excavation fleet he had already sent to Earth commanded exclusively by Brutes, but he would also leverage the abandoned Brutes who had served in Regret's initial fleet.
Aboard those vessels, the Brutes were ready and willing to unflinchingly obey the Prophet's commands. Once Truth gave the word, these soldiers forcefully overthrew the Elites and assumed complete control of the battle above Earth. Upon further orders from Truth, they aggressively deployed significant infantry directly into New Mombasa and picked up where Regret had left off while holding the human forces at bay until Truth's reinforcements arrived.