The humans have a large, long, parade to celebrate Earth Day. What is Earth Day? It's a day where they remember their homeworld, the beauty of it, how important it is as the starting point of their civilization, and their renewal of a promise to try to avoid the mistakes of the past, lest they ever again risk their future. -- Anon Guest
The assembled colonial polities of Terra, universally known as the United Fellowship of Terran Planets, synced their disparate calendars to this one day. Specifically, it was synced to Earth's June 25th. Earth Day. Those who participated gathered representatives, displays, and so forth to present as a united front.
In the time before the Shattering, some lip service was played to the planet's needs, and some people attempted to do more than that. Thankfully, time has changed it into a celebration of love for their old world.
The confetti contains flower seeds. Non-toxic, lest birds eat them. The streamers, if lost to the parks and landscapes, will sprout bee-friendly blossoms. Some of the floats will later become parts of the reefs, and homes for the fish. The people marching, or playing roles on the floats, will each plant a tree on a relevant patch of land. This is how Humanity remembers its greatest mistakes.