Arbor Day has just as much cultural significance as Christmas does on certain worlds.
A mountain is raised or lowered, or a valley filled in.
A network of fusion reactors are built to fuse the nitrogen in the atmosphere into oxygen, and bio-engineered plants are seeded that produce oxygen by converting the carbon dioxide through respiration, and spreads quickly, like Kudzu does; may be a Multipurpose Monocultured Crop. Processor drones are released to condition the soil. Oceans are created by melting the polar ice caps and/or dropping comets. Once the planet has the same atmospheric composition that earth does, the fusion reactors are recalibrated to produce power instead of air and some of the bio-engineered plants cleared off and converted into fertilizer for farmland. The conditioning drones are recollected and refurbished for later use. That being said, the planet needs to be within certain parameters before the work can begin- 1-G gravity, inside the star's Habitable Band, soil quality, has an electromagnetic field, etcetera.
Earth is dying; without a planet to escape to, mankind is doomed with it, so humanity begins a massive effort to build a new home.
Earth is turned into an uninhabitable wasteland by an alien race trying to reproduce their homeworld's environment.
Double Subverted: ...Until the colonists decide that would make all they've invested in terraforming the planet mean nothing, and decide to take good care of their new home.
The planet is made livable by spraying it with grass seeds from orbit.
A desolated area is reclaimed by importing butterflies and bunny rabbits.
Zig Zagged: ???
Averted: Turning a rockball of a planet into an earth-like one is too energy, resource, and time consuming, so it's simply left barren while the survey fleet looks for planets that are already earth-like.