I've been to Castel Gondolfo and it's a spectacular sight.
But I don't know anything about a sign of Mary's Assumption. First I've heard of that.
Mary is considered the precursor of us all, in some ways. In Catholic doctrine, she was conceived without sin, meaning that she was not marred by Original Sin. Her death, therefore, was different, because it was like the death of someone before the Fall.
For Catholics, this is a restatement of the reality of eternal life, the goal of earthly life, and the fact that Mary (who was not God or part of the Divine Nature) has gone before us and will be helping us from Heaven, as any good mother - or even older sister - would do. Catholics see the saints as their older brothers and sisters, who are not dead but have gone before and will intercede for them. The reason Catholics can address the saints and pray for and even address the dead, who may or may not be in Paradise, is that death is no more, and in eternity, we are all alive in Christ.