On a prograde planet like the Earth, the sidereal day is shorter than the solar day. At time 1, the sun and a certain distant star are both overhead. At time 2, the planet has rotated 360° and the distant star is overhead again (1→2 = one sidereal day). But it is not until a little later, at time 3, that the sun is overhead again (1→3 = one solar day).
An apparent sidereal day is the time it takes for the Earth to turn 360 degrees in its rotation; more precisely, is the time it takes a typical star to make two successive upper meridiantransits. This is slightly shorter than a solar day. There are 366.2422 sidereal days in a tropical year, but 365.2422 solar days, resulting in a sidereal day of 86,164.09 seconds (or: 23 hours, 56 minutes, 4.09 seconds).
The reason there is one more sidereal day than "normal" days in a year is that the Earth's orbit around the Sun offsets one sidereal day, giving observers on Earth 365 1/4 days, even though the planet itself rotated 366 1/4 times (the Earth rotates in the same direction around its axis as it does around the Sun: seen from the northern sky, anti-clockwise).
A civil clock day is typically 86400 SI seconds long, but will be 86401 s long in the event of a leap second (or possibly 86399 s in the event of a reverse leap second, but a reverse leap-second has never happened yet).
In astronomy, the siderealday is also used; it is about 3 minutes 56 seconds shorter than the solar day, and close to the actual rotation period of the Earth, as opposed to the Sun's apparent motion.
Days such as Christmas Eve, Hallowe'en, and the Eve of Saint Agnes are the remnants of the older pattern when holidays began the evening before.
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