A transit of Uranus from Neptune occurs when Uranus

Uranus

Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...

 passes directly between the Sun

Sun

The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

 and Neptune

Neptune

Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...

. Uranus can then be seen from Neptune as a small black disk edging slowly across the disk of the Sun. A central transit lasts about 42 hours.

This is the rarest of all transits involving the eight planet

Planet

A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...

s, owing to the long synodic period of 172 years (of Uranus from Neptune) and the very small apparent diameter of the Sun (1.07 arc-minutes) as seen from Neptune. And the mutual inclination of the two orbits is 1.5 degrees, which is less than that of most planetary considered pairs.

Transits seen from Pluto

Pluto

Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...

are even rarer, due to the inclination of its orbit, compared to each others.

The next transit of Uranus from Neptune will occur in October, 38172 CE.