The Regolith X-Ray Imaging Spectrometer (REXIS) instrument onboard NASA’s OSIRIS-Rex sample return mission was designed, built, and operated by students in collaboration with engineers and scientists at MIT and Harvard (see Allen et al., Hoak et al., and Masterson et al. in this conference). REXIS is a wide-field coded-aperture telescope with a 2x2 array of CCID41 X-ray CCDs sensitive to soft X-rays in the 0.3–8 keV band. REXIS was designed to measure X-ray fluorescence from the surface of Bennu stimulated by incident solar X-rays. For self-calibration, REXIS is equipped with a set of iron-55 radioactive sources to monitor changes in the spectral gain, as well as the offset and quantum efficiency of the X-ray CCDs during the journey to and observations of Bennu REXIS also carried out a series of calibration operations through observations of celestial sources including the Crab Nebula and Sco X-1. We review the performance of REXIS calibration activities and lessons learned for planetary X-ray fluorescence instrumentation utilizing X-ray CCD arrays.