Running from December 19th to January 9th, Destiny 2's event starts and ends later than many of its peers. In this yearly winter-themed holiday festival that heralds in the new year, there will be a number of different in-game items you can obtain to help put you in the holiday spirit.
In the Tower and the Farm, there will be snowballs scattered around which you can tag your friends with. In Strikes, these same snowballs can be used to stun enemies and deal damage.
The popular Crucible mode, first introduced in Destiny 1's The Taken King expansion, is seeing a return. This mode has abilities recharging much faster and makes kills and other actions worth more points. It's unclear whether or not Mayhem will be sticking around after the holiday event is over.
Each of the 3 weeks of the event, Tess will have new Dawning themed items. Over the course of the event, all of the exotic Dawning themed items-- ships, ghost, sparrow, emote-- and every Dawning-themed legendary armor piece will be available at her store.
There will be milestones you can accomplish during the Dawning for both strikes and the Crucible's Mayhem mode.
There will also be new consumables that give boosts to everyone else you are playing with.
You can also pick up a Dawning gift schematic from the Bazaar. Crafting and gifting said schematic can give you a number of potential gifts in return. You can repeat this daily.