Today is Raksha Bandhan, a day to celebrate the brother-sister relationship. Raksha means protection, and Bandhan means tie. Sisters tie a rakhi (symbolic thread) around the brothers' right wrist and in return, the brother vows to look after her and protect her.
Really, it can be anyone that you think of as a brother/close friend. I don't have any brothers, but I always tie rakhis to my four cousins (or at least try to send them one when we are on different continents).
Lots of pretty rakhis
Sadly, like most festivals, Raksha Bandhan has become mired in consumerism. But I'm not going to delve into that. I think it's a wonderful celebration. I already tied one of my brothers' a rakhi. He promised to protect me, and very sweetly, gave me some chocolate too. :)