The Aurbathimoovar or the Festival of the 63 Saints occurs in the Tamil month of Panguni and when the Moon transits the "asterism" or "star" or "nakshatram" (the Tamil word for star) called Uthiram during the Tamil month of Panguni i.e. March - April. This is also known as the Meena - Uttara Phalguni in Sanskrit. This transition occurs on the full moon day in the month of Panguni which is the 12th month in the Tamil calendar. The month is special because the Uthiram nakshatram coincides with the full moon.
The full moon on this day signifies the marriage of Shiva with Parvati, Murugan with Deviayni, Aandal (also known as Kothai) with Rangamannar etc. Thus this great celestial event of the marriages of the various Gods and Goddesses is celebrated with great pomp and pageantry.
The Aurbathimoovar or the Festival of the 63 Saints or the festival of the 63 Nayanmars. The Nayanmars were saints who dedicated their lives in the worship of God Shiva as well as composing and singing hymns in praise of Shiva. While the average Tamilian may not know the names of all the 63 saints he/she would certainly know the prominent amongst them such as Appar, Sundarar, Tirugnanasambandar and Kannapa Nayanar.
This festival marks the celebration of these saints who are taken out in procession in chariots and it their way of showing respect to the presiding deities of the Kapaleeshwara Temple in Mylapore, Chennai.