"Raksha Bandhan" (the bond of protection ) is a very popular festival of India.
Sisters tie "Rakhi" or holy threads on the wrist of their brothers, to ensure
protection from all evil obstacles and wishes for their well being. The brother
in turn acknowledges the love with a promise to be by the sister's side through
the thick and thin and gives her a token gift. Sweets are eaten thereafter to
celebrate this special bonding.

Tying rakhis are not always restricted to a brother. One can tie rakhi to a
sister, cousin, or even an adopted brother. You can even wish or tie rakhi
on the hand of a friend.

This festival is celebrated across India on the auspicious occasion of Rakhi Purnima,
or the full moon of the month of Shraavana.

More info here.... http://en.wikipedia.org/wiki/Raksha_Bandhan