The year 18,780 BBY (Before the Battle of Yavin) falls within the Galactic Republic's Great Manifest Period. This era, which had been ongoing for over 1,000 years, was characterized by the expansion of settlements from the Core Worlds into the "eastern" regions of Wild Space relative to the galactic center. On the planet Shawken, located in the Core, an iconic structure that extended into low-orbit within the atmosphere was obliterated. This structure was later rebuilt, and its destruction and subsequent reconstruction were commemorated by the historian Vicendi almost 9,000 years afterward.
The construction of what would become the Shawken Spire was initiated by inhabitants of the Core Worlds planet Shawken at least three millennia before the founding of the Galactic Republic. Over the following 9,000+ years, the Spire became increasingly immense. Simultaneously, the Republic was founded and unified and experienced growth through settlement and annexation during the Expansionist Era and the early stages of the Great Manifest Period.
By the time 18,780 BBY arrived, the Great Manifest Period had already been underway for more than a millennium. The Republic pressed onward with its eastward expansion into the unexplored Wild Space regions that would eventually be known as "the Slice." Situated in the Core, the Shawken Spire had evolved into a towering edifice that reached the altitude of low-orbit. Constructed from ancient materials, the structure had become famous across the known galaxy. However, this iconic structure was destroyed at this time.
The Republic's expansion continued for nearly another 2,000 years as the Great Manifest Period continued. The famous Spire on Shawken was later rebuilt using more modern materials. Nevertheless, the original structure maintained its prominence and was included in a catalog of galactic wonders created by the historian Vicendi in his 10,000 BBY publication, Arturum Galactinum.
The only canonical reference to the year 18,780 BBY appears in The Essential Atlas, in connection with the destruction of the Shawken Spire. The year is also indirectly alluded to through the Atlas's designation of the Great Manifest Period as spanning from 20,000 BBY to 17,000 BBY.