Before the age of hyperspace jumps and the ubiquitous use of publicly-maintained wormholes for stable warp travel, spacefaring was much lower-tech, and because it required ships capable of traveling at relativistic speeds, and crews who understood what that would mean for their planetside social lives, it was a profession entered by few and departed by fewer. After traveling with the same crew for long enough - and when it's three months on the inside, and two years on the outside, at 99.2% of c, "long enough" happens pretty fast (pun intended) - a 'naut would start to view her crew as the only familiarity, the only "home." This brewed its own ship-internal culture, which included a different timekeeping standard from the terralubbers.
The oldest recorded ship's calendar belonged to the NFTL (Nearly Faster Than Light, which is to say Not Faster Than Light) vessel Skedaddler, owned by the Day Deferred Trading Company, who specialised in transporting fast-spoiling delicacy foods, still fresh and intact thanks to special relativity, to future formal events on far-flung planets. The Skedaddler crew realised pretty fast that counting the days one by one was a bit of a wasted effort: as the ship accelerated to light speed, each day on the inside equated to a consecutively longer span of relative time on the outside. Likewise, as the ship slowed back down from near-c to make port, each passing shipside day was a little closer to lasting the same duration as a planetside day. Instead of counting the days by sunrises, which didn't exist (and were not yet simulated on NFTL freight couriers), hours were counted by the rotation of the watch who kept peace on the ship, and days were counted by what fraction of the total journey had been completed.
Launch day out of port was called the Kalends, because Captain Bianca Zanetti had a strong classical education and an odd sense of humour, and the rest of the calendar followed from there: the first quarter of the journey ended with the Nones, which marked the point of no return on the ship's acceleration. Until the Nones were reached, the ship could reasonably slow down enough to give and accept emergency aid with other vessels, and it could make port at one of the other worlds en route, for more serious concerns that required completely interrupting the journey. After the Nones, not only was the ship traveling so fast that the exterior time to slow down would be prohibitive to get help or give it, but also by this point the ship would be too far removed into the planet-empty middle of the trade route, with nowhere reasonable to stop along the way. The Nones were celebrated with a higher quality of rations, issued just for that specific day.
The Ides marked the middle of the journey, the point at which deceleration - and consequently, the countdown to make port - began. If the Nones were a celebration, then the Ides had a certain somber quality about them: having just made it halfway, the crew knew how much longer to the day they'd need to keep breathing the same recycled air and drinking the same recycled water, which by now had a certain stale alkaline taste to it, which (if crew diaries are to be believed, though this historian suspects it was largely psychological, what we now would call cabin fever) the filtration units of the time could not remove effectively. Any interpersonal disputes and drama which had arisen in the past weeks or months of transit, would now need to be kept contained again for just as long. Captain Zanetti chose to exorcise these social tensions by instituting a ritual which made the Ides of Space both playful and sobering: arming her crew with plastic dining utensils from the galley, and donning a layer of body armour and a trashed old EVA suit saved for the purpose, she invited the crew to stage a mock-mutiny and assassination of the captain. The crew used up their aggressions all at once, and once they had tuckered themselves out, the Captain would take off the reflective EVA helmet, which allowed her "assassins" not to see her face, and she would give a speech on the importance of cooperation and patience with each other. To fight among themselves was just as bad as ganging up on the Captain with lethal intention, because space has no patience and does not cooperate; every last bit of it is ready and waiting to - quote - "Murder your damn face off."
The last part of the ritual appears to have been a fusion of traditions brought in from American Thanksgiving and Yom Kippur by crewmen, approved by Captain Zanetti. The Ides concluded with a meal the crew collaborated on cooking together, using their hands and real fire (yes, I know! I was horrified, too, when I read it!), and they would take turns carrying plates full of food to each other, seeking reconciliation over the pre-Ides tensions.
Everything past that point was not necessarily smooth sailing, but it was still a countdown to the next shore leave, and if the Captain's accounts are truthful, the Ides' ceremonial violence and forgiveness really did mitigate crew disunity in the deceleration half of the journey.
Iron Noder 2019, 19/30