The Calm Lands are a massive, open area that the team reaches after traveling through Macalania Woods. It is home to the Monster Arena, the Cavern of the Stolen Fayth and Remiem Temple.
Technically your team only has to travel through the Calm Lands one time, but the Calm Lands are involved in a number of other side quests. The game will take you through the Calm Lands after the second visit to Macalania Woods. Click on the link below for more information:
There are a ton of side quests that involve visiting the Calm Lands but these side quests take place in the areas that branch of from the Calm Lands. These include the Monster Arena, the Cavern of the Stolen Fayth and Remiem Temple. Click on the links below for more information:
Cavern of the Stolen Fayth
This side quest can be completed as soon as you reach the Calm Lands and is where you obtain the optional Aeon Yojimbo.
Remiem Temple
Obtain the Celestial Mirror and battle all of Belgemine’s Aeons to receive the Moon Sigil required for Yuna’s Celestial Weapon, Nirvana.
Use the menu link for more information about the Monster Arena.
You will also have to visit the Calm Lands to obtain Tidus’ Celestial Weapon, 
Caladbolg. Click on the link below for more information:
