Historical information stored in the software system logs, audit trails, traces of user applications, etc. can be analysed to discover the patterns in periodic variations of levels and structures of the past workloads. These patterns allow for the estimation of intensities and structures of the future workloads. The correctly anticipated future workloads are used to improve performance of software systems through appropriate allocation of computing resources and through restructuring of associated system support. This work defines a concept of periodic pattern and presents the algorithms that find the periodic patterns in the traces of elementary and complex operations on data recorded in the system logs.