Today is the Thirunakshatram of Periya Perumal (i.e. Sri Ranganathar in Sri Rangam). Periya Perumal is the first in the Oran Vazhi (Guru Paramparai).
Here is a small history about Periya Perumal in Sir Rangam. Lord Ranganatha was the thiruvaradhana perumal of Lord Sri Rama. After the battle with Ravana, Lord Sri Rama’s pattabishekam was done. Sri Rama presented gifts to everyone who assissted Him in the battle. Out of affection for Vibeeshana, Sri Rama gave him his thiruvaradhana perumal, i.e. Sri Ranganathar. While coming back to Sri Lanka, Vibeeshana want to do sandhyavadhanam and so kept the Idol near tha banks of river Cauvery. After doing the sandhyavadhanam Vibeeshana tried to lift Sri Ranganathar, but couldn’t. He heard Sri Ranganathar’s voice stating that he wanted to stay there. Vibeeshana was worried, but Lord Sri Ranganathar blessed him and said that he would stay facing his country (i.e. Sri Lanka). So Sri Ranganthar faces South in the Sri Rangam temple.