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 second is a unit of time used by astronomers and is derived from the SI unit system. We have used the symbol s-sr.
1 sidereal second is 1/60 of a sidereal minute. This is derived ultimately from the sidereal day which is the time taken (in solar seconds) for the Earth to complete one rotation with respect to a distant star or constellation.
Like with normal seconds, minutes, hours and days, there are still 60 x 60 x 24 seconds in a day, but the day itself is approximately 23 h 56 min and 4.1 s in (normal) seconds.