Jupiter -- the largest planet in a solar system -- takes 11.86 Earth years to perform one complete orbit of the Sun. In this time, something happens twice: three of the planet's four brightest and largest moons (known as the Galilean moons, Ganymede, Callisto, Io and Europa) out of a confirmed 67 will pass between the Sun and Jupiter at the same time, casting their shadows onto the planet's face -- and the phenomenon will be observable from Earth.
At 4.36 a.m. GMT, on January 24, 2015, a multiple lunar transit will commence. Callisto, whose shadow will have started crossing Jupiter at 3.10 a.m. GMT, will be joined by Io -- but double lunar transits are pretty common. The real magic begins at 6.26 a.m. GMT, when Callisto and Io will be joined by Europa. All three shadows will be visible on the face of Jupiter until 6.52 a.m. GMT.
That won't be easy for everyone to see -- in Sydney, Australia, for example, that will be 4.30 in the afternoon -- but something else is occurring with the Galilean moons throughout December: they're currently aligned in such a way that astronomers on Earth -- professional and amateur alike -- can observe a number of occultations and eclipses between Ganymede, Callisto, Io and Europa.
You can find out when these will be occurring by using the interactive Global Sky Almanac tool recently launched on Astronomy Now. It allows you to enter a date and a location to see what events will be visible in the sky over your head at that time. You can also check out this post for more details about the phenomena.