Samsung will likely announce its next flagship, the Galaxy S9 and Galaxy S9+, at the Mobile World Congress in Barcelona next year. While a report from Korea claims the announcement will take place on February 27, a more plausible date seems to be February 26th.
While MWC 2018 officially starts from February 26th, all major press announcements will take place a day before (i.e. on February 25th, Sunday). So, it is more than likely that we will see Samsung unveil the Galaxy S9 on February 25th rather than on Feb. 26th provided the company decides to go ahead and unveil its next flagship at MWC.
Samsung has introduced a number of its flagship handsets at the Mobile World Congress previously but last year after the Galaxy Note 7 debacle, the company decided to postpone the launch of the Galaxy S8 to a separate Unpacked event held in late March.
The Galaxy S9 is expected to sport modest changes compared to the Galaxy S8 at least in terms of design. Samsung will be repositioning the camera setup vertically at the rear which would also see the fingerprint scanner move to a lower position thereby making it more ergonomic friendly. The internals of the device, however, will see massive improvements with the switch to a faster Snapdragon 845 chip, 6GB RAM, 64GB of UFS 2.1 storage, and a microSD card slot. Samsung is also looking to differentiate between the Galaxy S9 and Galaxy S9+ by including a dual-camera set up on the latter.
Update: A Bloomberg report also claims that Samsung will be unveiling the Galaxy S9 in February followed by a launch in March.