"December 1st, 2012" is an objective in Assassin's Creed III you play as Desmond
Walkthrough
Go on forward and get to the stairs ahead on the left. Keep going until you reach the outer part of the ticketed area. Rebecca will suggest stealing a ticket since they don't have the time to play nice. Break off to the right and go on through the open door to the right. Head on through the small garden.
There are some guards here so duck into the bushes to get out of sight. Sneak toward the next door using the bushes:
Wait for the guard to clear away then move on in and through the door. There's at least a chance you will get notice. Time for blending then ducking into the bathroom. You must blend but for once without the aid of the Animus to show that you are blended. As only as you have at least 2-3 people then you can blend. From there, make a break over to the restrooms.
Once in the restrooms go for the exit in the back left. Head on up the stairs and go to the right. Head for the Exit area before the VIP zone. This will let you get past the checkpoint:
Head on up the stairs and slide underneath the partly open shutter. Go on up the railing and over to the right. Climb the fence on the left. Jump for the pipe ahead of you. Climb up onto it and jump to the top of the garage entrance. Follow that by going over to the right and through the open window. Fast walk on through the corridor with all the people. Be careful to make your way to the end of the corridor as there is a guard patrolling there. With all of that done you need to head for the other side of the stadium.
Make your way along the scaffolding. Follow the yellow wire on the scaffolding to make your way over to the opposite booth. Slip on through the room and out to the corridor beyond. Go on over to the left to keep moving toward your goal. When you get to the next booth you will find Cross there and he's already killed all the people in the box. He has the artifact. Time for a chase.
Head on down the stairs, jump the barricade and keep on after him. This will eventually lead you here, back to the garden:
Now you need to take down Cross and the guards who've come out with him. The guards are easy, but take them out first to avoid them shooting you. After that it is a lot easier to deal with Cross. Take him down with a good punch string to finish out this mission.
Time to escape via the metro. Start following the signs and just run. Head through into the terminal and turn to the left. Head for the metro entrance there and run on down the stairs. Break to the right immediately and head for the bottom of the stairs to finish this on out and get back to your base of operations.
Place Artifact 2
The Second Power Source now needs to get in place. Head to the door near Rebecca's work station. Start up the broken staircase and take it upwards. Keep going up until the next cutscene. This is where it gets tricky. Go to the left at the top of the stairs and into the larger room on the right. Run to the last "window" and drop down. Below that you will find a small room you can go into.
Once inside just climb on upwards and go on forward to find the console to put the power source in. After that return to the Animus and get back to Connor's life
