In a one on one ROME REPORTS interview, Cardinal Tarcisio Bertone, says that in mid 2012, Benedict XVI told him he was thinking of resigning. For months, he kept it a secret, until it was announced to the world on February 11, 2013.
CARD. TARCISIO BERTONE
"Pope Benedict XVI thought about this decision long before announcing it. He told me about it in mid-2012. I told the Pope all the problems that could arise if he did so. But, he felt tired, his age weighed on him. He was concerned with World Youth Day in Rio de Janeiro. He would ask himself, 'At my age, how will I, address millions of young people?' As he explained later on, on February 11, he felt that in order to fully carry out the Petrine mission, he needed more physical and mental strength. He personally explained it, but I would tell him, 'Look, you still have to complete the trilogy on Jesus of Nazareth, and finish the volume on the Childhood of Jesus. It would be a great Christmas gift from the Pope to the people of God. Then, there's your Papal encyclical on faith, which you're still working on and plus, the Year of Faith just started.' But Benedict XVI decided that the date of his resignation would be February 11, 2013, the Feast Day of Our Lady of Lourdes.”
During those months, Benedict XVI's Secretary of State, guarded this secret which was somewhat unthinkable for the rest of the world, until it was announced on February 11, 2013.