It has been pointed out that the Galactic satellites all have a common mass around 107 M sun within 300 pc (M 0.3), while they span almost four orders of magnitude in luminosity. It is argued that this may reflect a specific scale for galaxy formation or a scale for dark matter clustering. Here we use numerical simulations coupled with a semianalytical model for galaxy formation, to predict the central mass and luminosity of galactic satellites in the Λ cold dark matter (CDM) model. We show that this common mass scale can be explained within the CDM scenario when the physics of galaxy formation is taken into account. The narrow range of M 0.3 comes from the narrow distribution of circular velocities at the time of accretion (peaking around 20 km s–1) for satellites able to form stars and not tight correlation between halo concentration and circular velocity. The wide range of satellite luminosities is due to a combination of the mass at time of accretion and the broad distribution of accretion redshifts for a given mass. This causes the satellites' baryonic content to be suppressed by photoionization to very different extents. Our results favor the argument that the common mass M 0.3 reflects a specific scale (circular velocity ~20 km s–1) for star formation.