Blizzard has yet to officially announce anything, but it seems that the Overwatch Twitter page in Brazil accidentally published a 20-second trailer confirming the upcoming event. The post has since been taken down, presumably as it went live earlier than Blizzard had wanted. Based on this it seems that an official announcement is imminent.
As to what players might be able to expect from the second anniversary event, based on the trailer it looks like new skins will be part of it. The trailer revealed Junkrat in a green pirate skin which is completely new, so there’s that. Presumably other characters will also be getting new skins, so not to worry if Junkrat isn’t your main.
In any case like we said, an official announcement should be coming soon so we’ll be keeping our eyes peeled for more information when that does happen. In the meantime you can check out the post on Reddit where the video was saved.