Thiruvathirai or Arudhra Darisanam / Darshan is observed in the Tamil month of Margazhi (December - January), on the day when the Arudra star and the new moon day coincides.
Arudra / Ardra is the name of a star (Nakshatra). It is the birth star of Lord Shiva (Rudra) and the Tamil poet Maanikavasagar who wrote Tiruvasakam, the key religious text of Tamil Shaiva Siddhanta.
Arudhra Darshan is a festival dedicated to Lord Nataraja (dancing form of Lord Shiva). His consort is Goddess Sivakami Sundari. The term 'Nataraja' means 'Lord of the Dance' or 'King of Dancers' (Nata = Dance; Raja = King). Lord Nataraja is considered the supreme lord of dance. It is believed that Lord Shiva incarnated in the form of Lord Nataraja during the Arudhra Darshan day. Arudhra Darshan is the most auspicious day like Mahashivaratri for the Saivites. This day is celebrated as Thiruvathirakali in Kerala. It is a dance performed by women in order to attain everlasting marital bliss. Married women fast on this day for their husband's well being and the unmarried women fast to get a good husband. In Tamil Nadu, on this holy day Thiruvathirai Kali and a special side dish (Kootu) which is made out of seven vegetables are prepared for offering to Lord Shiva.
The most important Arudra Darshan festival takes place at the Chidambaram (Chit = Truth, Ambaram / Ambalam = Space). Chidambaram Sri Thillai Nataraja temple is one of the Pancha Bhootha (five elements) Stalam representing space (Akasha). The Secret or Rahasyam of Chidambaram (Chidambara Ragasiyam), that God is formless and permeates everywhere. The Dance performed by Lord Shiva is known as Tandava. The tandava performed with joy is called Ananda Tandava and that which is performed in violent mood is called Rudra Tandava. Lord Nataraja performed Ananda Tandava (the cosmic dance of bliss) in Chidambaram temple. The Ananda Tandavam of Lord Shiva is said to symbolise the five divine acts (Pancha Krityas). Of the five celebrated dancing centres of Lord Nataraja, Chidambaram temple is Chittrambalam. It is noteworthy that the famous Vaishnavite shrine of Lord Govindaraja Perumal, among the 108 Divyadesams is close to Lord Nataraja shrine. It is said that Chidambaram Temple is at the centre point of the world's magnetic equator.
The small drum He holds in His hand signifies SRISHTI or creation, the lower right hand that offers protection signifies preservation, the upper right hand that holds the flame stand s for destruction, the foot that keeps down the asura Apasmara indicates obscuration and finally the perennial dance indicates ANUGRAHA or blessing, all these to bless the devotees.
The Natyanjali festival dedicated to the Lord Natarajar is celebrated every year in Chidambaram temple which is an opportunity for all classical dancers, from all over the world, to perform and to pay their tribute to Lord Nataraja. No Bharatanatyam performance can be complete without at least one dance in praise of Lord Shiva as Nataraja.
It is believed that a visit to Chidambaram Natarajar Temple leads to liberation.