A planet completing one full rotation on its axis is called a "day." This is what determines the length of a day on that planet.
one rotation of its sun
It is the rotation of the planet one complete revolution that determines the length of a day.
How long it spins on it's axis
A planet's rotation on its axis determines the length of its day.
A planet rotates on its axis, an imaginary line that runs from its North Pole to its South Pole. This rotation determines the length of a day on the planet.
A planet's rotation refers to the spinning motion of the planet on its axis. This rotation determines the length of a planet's day and creates day and night cycles as different parts of the planet are exposed to sunlight. Rotation is crucial for maintaining the planet's equilibrium and atmospheric conditions.
A complete turn of a planet on its axis is known as a "day." This rotation determines the length of a day on that planet. For Earth, it takes approximately 24 hours to complete one full rotation.
The planet's rotational period is the only thing that determines the length of one day. (mostly - it also is dependent on how fast the planet orbits the sun. If it orbits quickly, the "day" will be measurably less than the time for one full rotation.)
The rotational period, or how fast a planet spins on its axis, determines the length of a day on that planet. A faster spin results in shorter days, while a slower spin leads to longer days.
The concept of a "month" doesn't really apply to any planet except the Earth. On Earth the month is closely connected to the Phases of our Moon. A month is not like a "day" or a "year" which depend on times of rotation and revolution of a planet.
The time it takes for any given planet to make one complete revolution around its sun determines the length of its year.