Calcium belongs to the alkaline earth metal group. Within a group, the melting point decreases from top to bottom. This is because the as the atomic radii increases the outer valence electrons are shielded by the inner electrons and experience a lower nuclear attraction. As a result the bonds can be broken easily which lowers their melting point.
The three elements below Calcium ib Group II A which have a lower melting point are: Strontium (Sr), Barium (Ba) and Radium (Ra)