Is Calcium paramagnetic or diamagnetic? And why.

An isolated ##Ca## atom is diamagnetic, since its shows no unpaired electrons.
##Ca: [Ar]4s^2##
However, an element’s magnetic character does not depend solely on , it depends on the empty orbitals that are next to the valence orbitals as well.
Notice that you have a vacant 3d-orbital very close in energy to the 4s-orbital. This sometimes can cause the transition of an electron from the 4s to the 3d-orbital, which causes two unpaired electrons to occupy the aforementioned orbitals and induce a paramagnetic character.
Another important aspect to take into consideration when discussing ##Ca##’s paramagnetic or diamagnetic character is the fact that it has a metallic structure, which implies delocalized electrons.
Here’s an answer detailing ##Ca##’s metallic structure and its paramagnetic character posted by another contributor:
http://socratic.org/questions/why-calcium-is-paramagnetic-instead-of-fully-filled-orbitals