(Note - If you have to come to this point and you are missing the Jaguar Head, you have to go back. It does not matter if you have loaded Xibalba. Climb down the wall, get the bike and drive back almost at the beginning of the level, where you met the fork. Follow the left way of the fork and refer to the Unnamed Days walkthrough for details.)

[Checkpoint] Get the Stone Headdress from the pedestal on the left. Approach the hole in the outer stone wheel of the calendar and press the Interact button to place the Stone Headdress in the hole and raise a handle. Now place the Jaguar Head on the statue in the middle of the calendar. With both pieces into place, the calendar unlocks and you can use it.

Solving the calendar puzzle - The calendar consists of three stone wheels. The headdress handle rotates the outer wheel while the jaguar handle rotates the other two (middle and inner ones). Notice that each wheel has a symbol on it. The goal here is to rotate the wheels so all symbols are alinged.

To solve the puzzle, grab the jaguar handle. You will notice that you can rotate it in both directions. Rotate the handle clockwise. The middle and inner stone wheels will begin to rotate. Soon enough, the symbols on them will be alinged with the symbol of the outer wheel.

(Note - It is possible to solve the puzzle if you rotate the jaguar handle anti-clockwise. However, you will have to make a more complicated play with both handles until all symbols are aligned.)

When you do it correctly, a crypt in the Ball Court opens in the next cut scene. A huge statue with two snake heads rises up, the slopes are converted into fire-breathing serpent statues and the path between them retracts, revealing the exit.

[Checkpoint] Going back to the Ball Court - Exit the area with the calendar puzzle and climb down to the road. Get the bike and drive back the road. pass the two small buildings with the pressure plates and return to the Ball Court. Drive down the ramp and enter the next room.

In the cut scene, Lara brakes and stops a few inches from a pit full of spikes. Leaving the motorbike, she notices Mayan hieroglyphs on the wall, mentioning "The Place of Fear". She understands that she has just reached Xibalba, the Mayan land of the dead.