It's how the Senate works. First, the Majority Leader files a cloture motion (which is what he did last night for the DADT bill and the DREAM Act). Then, Senate rules say a certain amount of hours have to go by (which is why were waiting for a vote on Saturday). Then, a cloture vote is held, which requires 60 votes. The cloture vote is a procedural motion where the Senate decides if they want to proceed to floor debate on a bill.

If cloture is achieved (if there are 60 yes votes), then Senate rules require a certain amount of time for floor debate, followed by a final vote that only needs a simple majority to pass (51 yes votes). The final vote is the one that will be held on Sunday.