Credit: NASA/JPL/Space Science Institute
So Cassini has shown us eclipses of one kind or another enough times. Saturn is a lot bigger than its moons, so they're always throwing their little shadows onto its clouds, or wandering into the shade of their gargantuan parent.
But around Saturn's equinox, something new becomes possible. With the sun and Saturn's moons all on a similar plane, an observer on one moon could now potentially see one of the other moons blotting out their view of the sun. Which is what's happened here, with Enceladus casting its shadow onto Mimas.
There's a time-lapse video of the event that you can watch here.