See start & finish times here

The free weekend runs from March 19 through 22 and will be available on Xbox One, PlayStation®4 system, Uplay PC, Steam, and Epic Games Launcher.

I was able to obtain The Fate of Atlantis Episode 1: Fields of Elysium during the free giveaway back in August 2019. Will I be able to access it during the free weekend?

Yes, absolutely! You'll be able to access Fields of Elysium in the trial version of the game if you have previously obtained it.

See DLC deal posted here