A sidereal day is the astronomical measure of a day. There are two ways to think of this measure.

1) It is the time that it takes the earth to make one full 360 degree rotation on its axis.

2) It is the time it take for a object located on your meridian to reappear on your meridian.

A sidereal day is 23 hours 56 minutes and 4.09 seconds long.

Our calender system is based on a solar day not a sidereal day.