The Union was most likely the first of the polities founded specifically as a local government for the entirety of the Ecumenical Main. Experiments in unifying the Ecumenical Cluster had blossomed and faded before, but most of these are definitively known to have existed before the invention of the jump drive. The rest are known from dubiously-sized gaps in a set of contradictory lists. For the same reason, the Union is not currently known by any other name- some claim it was known as the "Union of Stars", others prefer a different appellation. Most of the history of the Union is unknown, but much of what remains is chronicled in the story of the interstellar calendar known as the Union Year.
Some five hundred years before the Ecumene was founded, the Union flourished. It had started as an alliance between a few nations, but by now was one of the strongest confederal powers in the cluster- strong enough to have a near-monopoly on advanced propulsion technology. Union ships primarily used wing drives for sublight propulsion, though theirs were of the "sail" design that hampers its use as a maneuver drive. Due to this near-monopoly, Union ships were also the first to scout out the other half of the Ecumenical Main. There, it discovered that the fact that holds true throughout the Ecumenical Cluster held true there as well- most systems have at least one habitable world, and half of those have life at least somewhat compatible with that within the Ecumenical Cluster.
The Union's flimsy central government sensed a great opportunity. By positioning itself as the main conduit for inhabitation of the Ecumenical Main, it wouldn't have to worry about another power usurping its authority- as much of the cluster that wasn't within the Union was aiming for- and it would be able to control the likely-lucrative cross-Main trade. In order to do that, it would need to have its own major supply outpost- and not just any supply outpost, but the best-equipped one. As such, the Union decided it would make a world along the Main its new capital.
The Union's government very much liked the idea of predictions- and one of the most important predictions it made was that of which worlds within the Main would become the most prosperous without major intervention. In order to find the ideal capital, it would average the orbital periods of these worlds. What it came up with was coincidentally very similar to the orbital period of the cool fourth planet in the second system out of the Ecumenical Cluster- a planet and star system they quickly named Concord. To inhabit Concord would be a difficult task, but the Union was prepared to spend quite a lot of money on it (the predicted returns were very large indeed). As such, infrastructure would need to be established to prepare for it.
The plan would make great use of the reserves of usable supplies in the first system along the Main. There, several large space stations would be built- these would accommodate the large amounts of people who would be needed to make a colony and interplanetary civilization in Concord system, along with the supplies required to make it work. Because the jump drive was at the moment only able to propel relatively light starcraft, many would be needed to make both journeys- as such, most ships would be launched with no crew and the bare minimum of supplies (to be delivered by other ships built expressly for the purpose). The plan was conducted mostly in secret, with rumors being the only available information about it.
The Union next drummed up support for a unified calendar for the entire cluster, both by overt and by covert means. This way, the Union's calendar- to be established based on the orbital period of Concord- would be more popular. This was an ancillary step towards becoming de facto authority of the Ecumenical Main- it was considered likely to endear newly-colonized worlds to the Union. The later intra-Union vote to make the Union Year the official calendar failed, but the vote was close- as such, much of the Union adopted it anyway.
The plan succeeded after nearly a decade of preparation and deployment. The station on Concord was established in one of the warmest places on the planet- a wide floodplain very near the equator. Its spaceport rivaled those on much better-established worlds, such that for several decades the Union's capital city was considered just a starport town. With this done, it was possible to put forward the Union's new official calendar. As follows-
A 360-day (though the actual day on Concord is closer to three-quarters of the averaged day figure) year, divided into three seasons. Each season was divided into two half-seasons, each officially named after one of the founding worlds of the Union (a crucial part of the calendar's acceptance was the ability to rename or ignore these time periods in actual observance); which were in turn divided into a total of twelve months (each nominally named after one of the other worlds of the Union). The twelve months were divided into half-months, each named after another major world of the Ecumenical Cluster. This bifurcation of the time period was helped along (at least on Concord) by the fact that orbits of Concord's two synchronous moons, whose actual orbits were actually somewhat shorter than the periods of a month and half-month, appeared to roughly match those figures when observed from Concord. Each half-month was divided into three weeks, which in most derivations of the calendar are not named- but several sources and uses of the calendar attempt to give names to all seventy-two.
The Union Year is one of the more common calendar bases in the Ecumene, though much of the Ecumene only uses it as a secondary, tertiary, or otherwise auxilliary calendar. The easy divisibility of the number 360 means that units from the Union Year and its variants are often used in other calendars, and several calendars for further-out worlds simply multiply the size of the calendar to fit their needs.