The day is a unit of time and is an SI-derived unit with the symbol d.
On Earth, it is defined as 86,400 seconds and is approximately the time it takes for the earth to complete a full rotation around its axis. In the earlier days, this was measured by waiting for a cast shadow to match a template drawn from the previous day's shadow.
There are 365 days in a year and, on average, 30.42 days in a month.
The unit day has many different variants; depending on what is used to measure the Earth's rotation. In a sidereal day (a rotation with respect to a distant star or constellation, not the sun), there is actually 4 minutes less than 24 hours in a cycle.
The sidereal day is a unit of time used by astronomers and is derived from the SI unit system. We have used the symbol d-sr.
Sidereal literally means "of the stars". Otherwise known as the "orbital period", it defines the time period taken for two point masses to orbit each other.
On Earth, we used the solar day to regularise time and this represents the period taken to orbit the sun. The sidereal day (or stellar day) is the time taken for the earth to rotate with respect to a distant star and equals 23 hours, 56 minutes and 4.0905 seconds (or 86164.1 s, 23.934 h).