According to the Radio Times website, the CSI Series Finale – Immortality will be airing on Tuesday, September 29th 2015 at 10pm.

From the Radio Times website:

In a feature-length final instalment of the crime drama, the team comes back together to help solve a catastrophic case that paralyses all of Las Vegas when bombs hit the casinos Catherine inherited from her father. Meanwhile, DB and Sara make decisions that will affect their careers and dominatrix Lady Heather returns. With William Petersen, Marg Helgenberger and Ted Danson.

View further details here at the Radio Times.