what is the closest constellation to saturn?

Tonight Saturn is in the constellation of Ophiuchus. Of course Saturn is about a million times closer to us than any of the stars in that constellation, so it isn't "IN" the constellation in that sense, but Saturn is seen in against the background of stars that make up the constellation of Ophiuchus. This changes over time as Saturn orbits the Sun, but it can only be seen in constellations along the ecliptic, which is the 12 traditional Zodiac constellations plus Ophiuchus.

There is no one constellation any planet is in, since they orbit the Sun and change positions over the course of their orbits. Right now Saturn is in Ophiuchus (the 13th ecliptic constellation, bastard sibling of the well-known Zodiac constellations), but in a couple of years it will be in Sagittarius.

.

.

Saturn can be found in the predawn sky between the constellations Scorpion and Serpent Bearer. Mercury, Venus, Mars, Jupiter, amd Saturn will all be visible in the predawn sky up until the 20th of February. Mercury will be close to the horizon though, so it will be difficult to see unless you are elevated or are in a flat area.

Constellations are entirely artificial man made creations. The result of our natural ability to see patterns. This results in us seeing patterns that aren't there. If the Solar System was is a different part of the galaxy, we would see a completely different set of constellations.

Constellations are apparent associations of stars at very different distances, so there is no distance from anywhere to a constellation.

Constellations are not physical things - you can't be closest to one.

Saturn, being a planet, moves around. Right now it's inside Scorpius.

Right now, it's just above Scorpius in the pre-dawn sky. Look a bit above and to the left of Antares, and you'll see Saturn.

At the moment it's in Ophiuchus, which is sort of in between Sagittarius, Scorpio, and Hercules.

If you mean on its path across the sky, that can be answered, if you mean physically, the "closest" is light years away.