The goal of the European Space Agency's Rosetta mission is a rendezvous with the Edgewood-Kuiper Belt comet 67P/Churyumov-Gerasimenko. After the initial encounter the spacecraft will accompany the comet for one to two years toward the Sun and throughout the perihelion passage. For the calibration of the onboard comet mass spectrometers a calibration system was built to simulate the neutral gas environment around a cometary nucleus. This facility consists of a fully automated three-chamber ultra-high vacuum system with a separate gas-mixing unit and a docking section with a five-axis table for instrument positioning and support. The gas-mixing unit produces mixtures of gases and also gas water vapor mixtures. The gas mixtures are fed into the vacuum system by a leak valve (static mode) or through a nozzle to form a molecular beam (dynamic mode). All flight and flight spare sensors of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) have been calibrated in the static mode of the Calibration System for the Mass Spectrometer Instrument ROSINA (CASYMIR). Preliminary calibration measurements in a pressure range from 10-9 to 10-6 mbar for carbon dioxide and for noble gases from helium to xenon show very promising results.