The Lanayru Spring is a location that appears in Twilight Princess. It is one of the four Light Springs located across Hyrule, including the Ordon Spring, the Faron Spring, and the Eldin Spring. Each spring is the home of each respective Light Spirit, thus meaning the Lanayru Spring is the home of the Light Spirit Lanayru. It can be found in Lake Hylia, to the west side.
Link visits the Lanayru Spring a mandatory amount of three times. However, Link can optionally return to the spring to claim a very helpful reward.
The first time Link goes to the Lanayru Spring is when the Lanayru Province is still in Twilight. He comes here to receive the Vessel of Light from Lanayru, allowing him to find the Tears of Light that restores the land to its former glory. This is a quick visit; he enters, listens to Lanayru talk about himself and Link's task, and then leaves.
Link's second visit is directly after he has compiled all the Tears of Light. It is here where Link restores the Lanayru Province back to light and heals Lanayru. Lanayru tells Link the tale of the Fused Shadows: how the Interlopers seized the Sacred Realm, and the Light Spirits were ordered by the Golden Goddesses to seal away them and their power into the Twilight Realm.
Link visits the Lanayru Spring a third time, right away defeating Morpheel in the Lakebed Temple. Midna warps Link to the spring, and the moment Link prepares to leave, Zant appears. He steals the Fused Shadows and seriously harms Midna. He also places the Shadow Crystal inside Link, causing him to be in Wolf form. He leaves with both Link and Midna seriously harmed.
Number 4 (Optional)
The fourth visit is completely optional and is only done in order to retrieve a Piece of Heart that lies in the area. Because it requires the Clawshot and for Link to be in Hylian form, it cannot be done until after the Lakebed Temple has been completed and the Master Sword has been obtained. Once those requirements are met, Link uses the Clawshot to reach a secret room in the back of the area, where he finds a chest that contains the Piece of Heart.