Every 134 standard months, the Harmony's comet visits every world on the fifth planet at the same time. The enlightened societies of those worlds are in good relations, not despite their differences but because of them. They use that cosmic event as an opportunity to celebrate their link.
The night in which the comet can be seen, representatives of every member of the Fifth Planet Family see it together, and there is a party that mixes traditions of every participant, even some that are now forgotten in their original worlds.
The main event is the exchange of promises, which often answer the needs of others, but sometimes are simply an open offer of help or good will. There may be a bit of empty poetry, but that doesn't seem to be an issue.
The full celebration varies according to the customs of the hosting world. They may receive the visitors up to four days before the actual Harmony Night, and show hospitality the best they can. Or they can prepare only the main ceremony and do little to accommodate the guests.
After the reorganization of Dhha, the temporary rulers decided to take their ideas to the neighboring worlds, starting the only successful process of intercultural relations of the discovery age. Despite the differences in development of planes, in 1911 of discovery age all of these worlds were visited by a comet in the same date and time. The celestial body was considered a symbol for some, and a sign for others, but the meaning was the same for everyone: peace in their relations. They decreed to name it Harmony, even when some of them had already assigned it other names. Back then, there was a party in Lien whenever the comet passed without causing harm, a way to show thankfulness to Nature. That year, they invited diplomats from the other worlds in the fifth planet to join the festivities and, in an improvised moving event, they all took the opportunity to exchange promises of allegiance and peace. It became a tradition, but the subsequent celebrations took place during the night when the comet could be observed, instead of the day after, and they take turns to be the host.