PlanSync

Planetary Synchronuous Low Delta-V Event more often called PlanSync is an event that rarely happen but is a great time to be alive when it does. While localised PlanSync, say Mars-Terra-Selene Sync is fairly common but still do bring some fesitvities or at least some opportunity for the population, larger Sync Event are rarer and thus often subject to festivities, or in some sad occurence War.

Nature

PlanSync happen when the planete or moon align in such a way that the Delta-v necessary to jump from one to another is significantly reduced, thus allowing for mission planner to send probes, ship or even Dvorak Class toward the other planete for the cheapest and smallest amount of time.
While not allign properly the planete are then the most "in sync" cause communication and commerce, or in some instance war, can happen at a larger scale than during the rest of the time.

Frequency

While inner planete sync are somewhat regular event -once every few year usually- outer planete PlanSync is very rare and some tend to happen on generational if not civilisational timescale.
While it is possible to fly to outer space outside of PlanSync event, it is considerably costlier and longer depending on the Sync index of the two body. Some travel are only economically viable and practically doable during high sync time and the resulting influc of commerce and availlable goods is the reason of festivities.

Culltural Significance

While Martian Republics and the CAH Colonies tend to see PlanSync as natural event that bring oopportunities and are more of a concern for merchant and engineers, Selene and United Earth Nations view it as a cultural significant event.

Earth

PlanSync, especially Global PlanSync is viewed as some kind of mystical and magical time. The UEN propaganda paint it as a time to let the past be gone and embrace "Unity through Purity" and press for using this time to "rekindle humanity spirit" in the heart of "thoses who are lost". While Straight up call for war to reclaim the moon or mars are usually nothing but empty threat, the UEN is indeed laying unusually aggressive diplomatic and propaganda effort during thoses times. SOmetimes with the clear intent to taunt adversaries into striking first.

UEN propaganda talk about the next Global PlanSync as the herald of the end of "dissention" and the time where all humanity will fold back under Terra rule.

Selene

Similarly Selene see PlanSync as a mystical and fundamentally religious event. They differ however in the way to achieve this unity and the PlanSync Festival is cause for sending countless automated preacher and talking head to the synched planete. They also have large festival of pleasure to "welcome home" any defector. This as proven to be a problem for other power, especially Terra who by sheer proximity is the most impacted