I'm not sure if it's
only on weekends, but I know it's not 365 days a year, unless something has recently changed at DLR. I asked the people at the Dining number about the buffet and was told that they do it seasonally - like during busy times of year (probably on assorted holiday weekends, during the summer, maybe during Spring Break, during the Christmas season, etc.) and some weekends. The last time I ate at Storytellers when the dinner buffet was happening was a Friday night in December 2008, smack dab in the middle of the holiday season.
So, based on what they told me at
Disneyland Dining, I'm not sure if you can count on the buffet being there every weekend all year long, or just during those specified times I mentioned above. I know some DIS-ers have reported the buffet being there when they went and others have said the buffet was not available at other times of year, so it's anyone's guess.
By the way - the dinner buffet (which is a non-character meal) is not very large. There are not too many items in it. If it's not available, you may just want to eat dinner at Storytellers anyway, and get something off the regular menu. Their spaghetti and garlic bread were absolutely delicious - the sauce had all kinds of fresh herbs and the garlic bread had chunks of fresh garlic. Yum!!