Severe turbulence is rare. I am a very frequent flier and have experienced it exactly once (in a thunderstorm). If your flight is scheduled to fly through the oval I have drawn, please keep your seat belt tight, don't get up more than necessary and don't eat a big meal right before take off.
Addition: I plotted a map (courtesy, NWS Aviation Weather Center) of the current reports of turbulence and their short term forecast of moderate or greater turbulence (brown tint). The area of turbulence will expand and increase during the afternoon to, more or less, the red oval above.