Tuesday is dedicated to Lord Hanuman. In South India, the day is dedicated to Skanda, aka Muruga or Kartikeya. Devotees also listen to Hanuman Chalisa, songs dedicated to the simian deity, on this day.
It is a whole day fast. Devotees who undertake Mangalvar fast only take a single meal usually consisting of any food made of wheat and jaggery. Most Hindus keep the fast for 21 Tuesdays without a break. Mangalvar Upvaas is to satisfy Hanuman (the monkey god) and Mangal (Mars) that rules the day, and from which it takes its name. Mangal is considered a trouble maker and the fast is observed to ward off evils and problems that strewn our path and make way for happiness. The underlying belief is that Lord Hanuman will help his devotees to overcome difficulties in life especially those created by the intervention of 'Mangal graha' or Mars. Also, couples desirous of having a son, too, observe this fast stringently.
Color & Gem:
People wear red-colored clothes on Tuesdays and offer red flowers to Lord Hanuman. 'Moonga' Red Coral is the preferred gem of the day.