Destiny 2 December Update: Here's What To Expect
The free anniversary event is going to include a new six-player activity called Dares of Eternity. Players can match with five others to "unravel secrets and [earn] rewards that commemorate [Bungie's] long and storied history." Apparently, the activity will be similar to Astral Alignment, and it will revolve around the Nine.

Also newly-available in the anniversary pack mission are other items that call back to classic Bungie titles, including a Thorn-inspired armor set, Destiny weapons like the Thousand Yard Stare, and a claymore sword from Bungie's classic PC RTS, Myth. There's also a Bungie Streetwear ornament set, a Marathon-themed ornamet set, and myriad other miscellany, including emblems, shaders, emotes, and so on.
Not to be found in the pack: anything to do with Halo. Halo is inarguably Bungie's most noteworthy property, but sadly, ownership of the game series is in Microsoft's hands, not its original creators. Technically we don't know that the update won't include Halo gear or goodies, but it does seem a bit like Bungie would be shouting from the rooftops about it if it did. As it stands, there's no mention of any such thing.
If it seems like the 30th Anniversary pack is just the thing to tide you over until the Witch Queen update, you can pick it up for $25 at Bungie.net before its release on December 7th.