Cession refers to (1) the act of ceding or surrendering something, and (2) something that is ceded. Session can refer to several things, including (1) a period of time devoted to a specific activity, (2) a meeting of a legislative or judicial body, and (3) a term at a school or university.
Both words only function as nouns, and they sound the same. Otherwise, they have no common ground. Still, they’re occasionally mixed up—for example:
[B]ut the large land sessions won by the Tribes through treaty in fact allowed future generations to survive. [Pak Tribune]
On June 16, Yerevan hosted the cession of a council for trafficking issues led by Territorial Administration Minister Armen Gevorgyan. [PanArmenian.net]
These are positive examples:
The 2011 Colorado legislative session was—on the whole—a good one for the environment. [Denver Post]
The treaty describes the Sioux land cession, about 100,000 acres. [The Circle News]
He watches my face go pale at the thought of a six-hour sprint session and cracks into a chuckle. [Evening Standard]
Paris finally fell and negotiations for a settlement began, ending unhappily for France a few months later with the cession of Alsace-Lorraine to Germany. [New York Times]
Evans overcame a frustrating qualifying session to give himself a great birthday present. [New Zealand Herald]