Sisters tie a thread on their brothers' wrists on the full moon of Shravana, and the brother owes protection in return. The whole obligation runs off a lunar date.

Rakhi threads are posted internationally in enormous numbers every year, which means a lot of international mail is timed by a moon. The custom has been extended in various directions — to soldiers, to neighbours, across religious lines — and the date has never moved off the Shravana full moon.