Erbium nitride (ErN) is a rare-earth metal mononitride with desirable electronic, magnetic, and optical properties. ErN can be incorporated into III-nitride semiconductors to develop new functional materials for optoelectronic and spintronic devices. Here, we report on the optical properties of ErN crystals, grown by sublimation and probed by photoluminescence (PL) spectroscopy. Three transition lines were observed near 1 eV. Theoretically, ErN has a small indirect energy gap of around 0.2 eV with a conduction band minimum at the X-point of the Brillouin zone and a valence band maximum at the Γ-point. The predicted smallest direct energy gap is around 1 eV, with two valence bands at the X-point. Using the PL results together with the reported calculations, a coherent picture for the band structure at the X-point for ErN crystals has been derived. Experimental results revealed that ErN has a minimum direct bandgap of 0.98 eV and a total of two valence bands separated by about 0.37 eV at the X-point.