Multicast is emerging as an important means of transmitting the same content to multiple receivers so as to efficiently utilize the overall network resource. We propose an opportunistic multicast scheduling algorithm that exploits time-varying channel conditions in time division multiplexing (TDM) cellular networks and maximizes the overall network throughput under certain resource fairness constraints among all groups in the system. The optimum transmission rate for each group is selected by taking into account the current channel conditions and average received throughput of each user to achieve the best tradeoff between throughput and fairness. We then prove the optimality of the multicast scheduling algorithm and show that our algorithm does not deteriorate any group's performance to improve the overall network throughput compared with the non-opportunistic scheduling algorithm. Through extensive simulations we demonstrate that our opportunistic multicast scheduling algorithm improves network resource efficiency under fairness constraints.