Strangely, astronomers don't know how long a day is on Saturn, because
they can't get a firm footing on the problem given the giant planet's
gaseous nature.
So they have long relied on radio measurements of the ringed planet's
magnetic field to help estimate the length of the day. But that doesn't
really work either, they realized, so estimates have remained loose. Now
the scientists at least have a better handle on this aspect of the
problem.

Read it HERE:
http://www.usatoday.com/tech/science/space/2007-03-23-saturn-strange-day_N.htm

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...and from that same link:

Thought of the day

A day on Earth is determined by how long it takes the planet to spin
once on its axis. That's pretty easy to measure, because Earth's surface
is solid. Just sit there for about 24 hours, 3 minutes and 56.55
seconds, on average, and measure the time between two sunrises. (That
works today, but eventually we will have more than 24 hours to get this
job and others done. A lot more. In a few billion years, a day will last
about a month! )

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