Lord Shiva wore the moon crescent on His head, making him grow for 15 days and decay for 15 days periodically

 daksha had twenty seven stars as daughters. All of them were married to moon. But the moon had special attraction towards rOhiNi. So all the other wives complained it to the father dakasha that their husband moon is not treating them fairly. daksha got angry and cursed the moon to lose its luminance (kala) day by day. Everyday the moon started loosing one luminance part (kala) out of his 16 parts. Afraid and ashamed moon disappeared into the ocean. There are many herbs that require the light of the moon to grow. Without moon, they were harmed. Consequently there was suffering in the world. To end the problem the celestials advised moon to take refuge in the Great Divine, Lord shiva. Left with only one kalA, he took refuge in Lord Shiva. Being the graceful Almighty, He wore the moon crescent on His head, making him grow for 15 days and decay for 15 days periodically.