Sidereal is an adjective that first popped up in the 17th century, and stems from the Latin word sidereus, which means "star." Anything that's sidereal has something to do with stars and constellations. And if you measure the days and weeks by the movement of the stars across the sky, that's called sidereal astronomy or sidereal time. But if you've got someplace to be, you'd be better off with a good calendar and a wristwatch.