It will start next week and similarly to the Halloween or Winter events, it will include news skins and vanity items.
Details are scarce, but the Year of the Roster event will start in Overwatch on January 24th. It was announced via this tweet:
https://twitter.com/PlayOverwatch/status/821900007649329153
As you see, lovely new skins are expected. No additional details are available at this moment, but it’s almost 100% sure that the event will include a unique mission similar to Lúcioball, Junkenstein’s Revenge or Mei’s Snowball Offensive.
We’ll update you when further information surface.