When Daksha cursed Chandrama.

According to Shivpurana Prajapati Daksha did Vivah of his 27 daughters with Chandrama. Chandrama most loved Rohini among all of his wives. His other wives didn't like this thing.

When their father was told this thing he asked Chandrama to treat all of his wives equally but Chandrama didn't listen so he got angry and cursed him to decay (क्षय).

Then Chandrama did Tapasya for Shiva to save him from this curse, Shiva after being impressed by his Tapa appeared in front of him.

When Chandrama asked Shiva to get rid of the curse Lord Shiva explained to him that he has done wrong and he could not get rid of this curse. But to save Chandrama from this curse Shiva placed Chandrama on his head and boomed him to face this curse only for 15 days and for the next 15 days he will grow again to the fullest.