Aadi Krithigai, or Karthigai, is a festival celebrated by Lord Muruga devotees, especially by Tamilians. It is celebrated on the Karthigai star in the Tamil Month Aadi. The festival is also referred as Aadi Krittika and Krithika. In 2008, date of Aadi Karthigai is July 27.

Legend has it that Lord Muruga or Skanda was born as six different babies and it was Goddess Parvati who joined the Six to create the powerful Muruga or Kartik, who is the commander of the army of the Devas. But before conjoining the six different babies were looked after six mothers – the six Karthigai stars.

Karthigai Nakshathram day is considered highly auspicious to worship Muruga and especially when the Krithigai star day in the month Aadi. Lord Muruga Temples in Tamil Nadu, Kerala, Sri Lanka, Malaysia, South Africa and Singapore have special rituals and poojas on the day.

The Tamil Month Aadi is a month that has lot of festivals. During the month of Thai(Jan-Feb) and Aadi(Jul-Aug) the Sun changes directions.

Aadi month is special for all Ammans. It is celebrated in a grand manner in all Amman temples (The Famous temples as well as not so popular temples). Sundays, Tuesdays and Fridays of this month are auspicious.Every Friday is celebrated as "Aadi Velli". The new moon day of this month is celebrated as "Aadi Amavasai".The pooram star of this month is celebrated as "Aadi pooram".The 18th day of this month is celebrated as "Aadi perukku" or "Pathinettam perukku". Since the rains are abundant in this month, the rivers are filled with water.

Kirthigai star of this month is celebrated as Aadi Kirthigai and is considered a very auspicious day for the worship of Lord Muruga.