The dates for the 100th episode and the season five finale are set. The 100th episode will air on April 15. The season five finale will air on May 13.
We come back from the hellatus on March 25 with Dead Men Don't Wear Plaid. We get all the remaining episodes straight through. That means no re-run or pre-emptions.
The 100th episode (episode 5.18) is called Point of No Return. No title for the finale has been announced, I think they are still writing it.
WOOHOO I said!
Source: Maureen Ryan's The Watcher.