A sidereal day is two successive upper meridian crossings by the Sun of the Vernal Equinox - of course and easier way to remember this is that a sidereal day is measured by a complete Earth rotation with respect to the stars (not the Sun which would be a standard Solar day).

Vernal Equinox is when the Sun passes the Celestial Equator between the northern and southern hemisphere.

Vernal Equinox is also known as the first day of Spring - or (around) March 21.

Midnight at the Vernal Equinox is 00:00 hours local Sidereal time.

It is also important to understand that Astronomer's often use different times than we are used to - for example our watches are in sync with the Sun. This means that one day is equal to Earth's rotation with respect to the Sun. This is called a Solar day. For more accuracy, Astronomer's use Sidereal time, Earth's rotation with respect to the stars. This means that the same constellation appears at the same place after a complete rotation of the Earth. When comparing a Solar day to a Sidereal day, subtract 4 minutes from each other:

Sidereal Day = Solar Day - 4 minutes

This is why the night sky is different depending on the time of year, they rise 4 minutes earlier each night. So why 4 minutes?

A complete circle is 360° and Earth orbits the Sun in 365.25 days, or about 1° a day.

An Earth rotation is also 360 which is 24 hours (or 1440 minutes). Divide 360° into 1440 minutes and there is 4 minutes left over. To match with Sidereal time, the Earth must rotate 1° more for the Sun to appear in the same spot day by day.

A Sidereal Month is also used, and is the orbit of the Moon with respect to the stars which is 27.3 days. A Synodic Month is the "standard" Moon orbit about the Earth - from full moon to full moon - which is 29.5 days.

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