Rakhi

Raksha Bandhan (the bond of protection in Hindi) or Rakhi (राखी in Devanāgarī) is a Hindu and Sikh festival, which celebrates the relationship between brothers and sisters. It is celebrated on the full moon of the month of Shraavana.
The festival is marked by the tying of a rakhi, or holy thread by the sister on the wrist of her brother. The brother in return offers a gift to his sister and vows to look after her. Indian history is replete with women asking for protection, through rakhi, from men who were neither their brothers, nor Hindus themselves. Rani Karnavati of Chittor sent a rakhi to the Mughal Emperor Humayun when she was threatened by Bahadur Shah of Mewar. Humayun abandoned an ongoing military campaign to ride to her rescue.

Origins

Draupadi and Krishna during the Rajsuya yagya - After Shishupal's death, Krishna was left with a bleeding finger. Draupadi, the wife of the Pandavas, had torn a strip of silk off her sari and tied it around Krishna's wrist to stop the flow of blood. Touched by her concern, Krishna had declared himself bound to her by her love. He further promised to repay the debt many fold. Many years later when Draupudi was about to be shamed by being disrobed in front of the whole court by her evil brother-in-law Duryodhana, she called on Krishna to help her, and he did by divinely elongating her sari so it could not be removed.

-- Wikipedia