From what we've been able to find, the rumors are being driven by the reporting of Nelson Bocaranda, a well-known Venezuelan journalist. Saying he had received calls from "friends," Bocaranda said that we would hear about Castro's demise in "72 hours."
That was about 24 hours ago. Today Bocaranda did not back down from his previous statement tweeting, "They're expecting the arrival of Juanita Castro from Miami. She was invited by her brother Raúl to make an 'important family announcement.'"
Of course none of this reporting likely matters: News in Cuba, especially about the health of Castro, is a closely guarded secret and nothing will be official until the government decides to release it.
When that happens, we'll probably hear it from either a televised address or through the Communist Party's official newspaper Granma.