We derive exact probability distributions for the strain ( ) at which the first plastic drop event occurs in uniformly strained disordered crystals, with quenched disorder introduced through polydispersity in particle sizes. We identify the first plastic drop in the system with the first contact breaking event and characterize these events numerically as well as theoretically. Our theoretical results are corroborated with numerical simulations of quasistatic volumetric strain applied to disordered near-crystalline configurations of athermal soft particles. We develop a general technique to determine the distribution of strains at which the first contact breaking events occur, through an exact mapping between the cumulative distribution of such events and the volume of a convex polytope whose dimension is determined by the number of defects N d in the system. An exact numerical computation of this polytope volume for systems with small numbers of defects displays a remarkable match with the distribution of strains generated through direct numerical simulations. Finally, we derive the distribution of strains at which the first plastic failure occurs, assuming that individual contact breaking events are uncorrelated, which accurately reproduces distributions obtained from direct numerical simulations.