This paper presents some generalities about the VNSCalendar system, a tool able to understand users’ voice commands, would help users with managing and querying their personal calendar by Vietnamese speech. The main feature of this system consists in the fact that it is equipped with a mechanism of analyzing syntax and semantics of Vietnamese commands and questions. The syntactic and semantic processing of Vietnamese sentences is solved by using DCG (Definite Clause Grammar) and the methods of formal semantics. This is the first system in this field of voice application, which is equipped an effective semantic processing mechanism of Vietnamese language. Having been built and tested in PC environment, our system proves its accuracy attaining more than 91%.