MONDAY marks 500 days since young William Tyrrell disappeared from his grandmother's yard and child protection advocates are calling for answers.
Tuesday is the 50th anniversary of one of the country's biggest cold cases - the disappearance of the three Beaumont children.
Child protection advocate Hetty Johnston said Australia Day was about celebrating as a nation but it was also a time to reflect on what was not great.
"What is not great is that 50 years on something as insidious as the abduction of a child can still happen in this country - that William, the little boy in the Spiderman suit could simply vanish, taken from those he loved," she said.
William disappeared from his nana's home on the NSW Mid-North coast at about 10.30am on September 12, 2014.
He was three years old and wearing a Spiderman costume and police believe he was abducted.
Ms Johnston said Australia Day would mark two tragedies - 501 days since William disappeared and the 50th anniversary of the disappearance of the Beaumont children.
The Beaumont children, Jane, 9, Kathleen, 7, and Grant, 4, disappeared from a beach near Adelaide on Australia Day in 1966.
The children caught a bus for the short ride to a beachside resort and never returned home.
Ms Johnston said someone who knew something had remained silent for 50 years.
Similarly, she said someone had remained silent about William's disappearance.
"Most importantly, on Australia Day we must say that it's unacceptable to abduct a child and unacceptable for those who know something to not come forward, to not make the call that might help find William and bring him home," she said.
Anyone with information is urged to phone Crime Stoppers on 1800 333 000.