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We calculate the Curie temperature of layered ferromagnets, chromium tri-iodide (CrI3), chromium tri-bromide (CrBr3), chromium germanium tri-telluride (CrGeTe3), and the Neel temperature of a layered anti-ferromagnet iron di-chloride (FeCl2), using first-principles density functional theory calculations and Monte-Carlo simulations. We develop a computational method to model the magnetic interactions in layered magnetic materials and calculate their critical temperature. We provide a unified method to obtain the magnetic exchange parameters (J) for an effective Heisenberg Hamiltonian from first-principles, taking into account both the magnetic ansiotropy as well as the out-of-plane interactions. We obtain the magnetic phase change behavior, in particular the critical temperature, from the susceptibility and the specific-heat, calculated using the three-dimensional Monte-Carlo (Metropolis) algorithm. The calculated Curie temperatures for ferromagnetic materials (CrI3, CrBr3 and CrGeTe3), match very well with experimental values. We show that the interlayer interaction in bulk CrI3 with R3 stacking is significantly stronger than the C2/m stacking, in line with experimental observations. We show that the strong interlayer interaction in R3 CrI results in a competition between the in-plane and the out-of-plane magnetic easy axis. Finally, we calculate the Neel temperature of FeCl2 to be 47 +- 8 K, and show that the magnetic phase transition in FeCl2 occurs in two steps with a high-temperature intralayer ferromagnetic phase transition, and a low-temperature interlayer anti-ferromagnetic phase transition.