On the 13th June 2017 Frontier announced that the 2.4 update will feature the return of the Thargoids. You can see the short promotion trailer for it below.
The release date will fall between July and September 2017. As described on the Frontier forum,
“The Return will play out differently to previous releases in Elite Dangerous: Horizons, with content and story beats affecting all Commanders releasing in the months following its initial release.
“As a story-centric update, showing too much ahead of time would spoil the surprise and make for a lesser experience for the community. We want The Return to surprise and shock you as much as possible, but that said, you can expect for further reveals and information about the future of Elite at various community and gaming events throughout the year, such as the upcoming Lavecon.
“David Braben talks about the history of the Thargoids, and about how 2.4 The Return will unfold, in the video we posted to our YouTube account:”
Further details will be added to this page as and when it is known.