August 13, 1917 ... The Children of Fatima were unable to keep their appointment with Our Lady.

So what happened?

Because they were unable to be at the Cova, did Our Lady appear to them where they were detained?

Nope.  She kept her appointment at the Cova - according to witnesses - but she didn't make a command performance where the children were detained.  Although she visited them later, on August 19, to make known her requests - which included preparation for the Feast of Our Lady of the Rosary.  (Remember, we can prepare as well, many people will be praying the 54 day novena of Rosaries, commencing August 14 or 15, concluding on the Feast of OL of the Rosary.

Here is the story of the August 1917 apparition ...

Under the pretext of providing his personal automobile, so that the children could travel safely through the crowds pressing around their homes, the civil Administrator or Mayor of the district in which Fátima was located, arrived in Aljustrel on the morning of August 13th. A previous attempt on August 11th to obtain the "truth" from the children having been unsuccessful, Artur Santos, an apostate Catholic and high Mason, had devised a scheme by which he would take them into custody and force them to reveal all. With a show of good will he now offered to take the three and their parents to see the parish priest, whom he claimed wished to see them, and then to the Cova. At the parish house he abandoned this ruse, and the parents, taking the children alone from there to the district headquarters in Vila Nova de Ourem, some 9 miles away. Here he tried bribes, threats of death and locking them in a cell with other "criminals" in order to get them to recant their story. It was to no avail. Despite their ages, their belief in the Lady and their courage was unshakeable.

Meanwhile, in the Cova at noon on the 13th the characteristic external signs of the Apparition appeared for the benefit of the crowd, the greatest crowd to that time. After they ended the crowd dispersed, as yet unaware of the trickery of the government.

[...]

As it was the Lady's plans were delayed slightly. On Sunday the 19th Lucia, her brother John, and Francisco, were grazing the sheep at a place known as Valinhos. It was located on the side of the same hillock opposite Aljustrel where the angel appeared twice, though a little farther north. At apout 4 o'clock, sensing that Our Lady was about to appear, Lucia tried unsuccessfully to get John to fetch Jacinta, until she offered him a couple pennies for the errand. As she and Francisco waited they saw the characteristic light. The moment Jacinta arrived the Lady appeared. - Source

What did Our Lady tell the children before she left?

She looked at them sadly, and told them:

Pray, pray very much. Make sacrifices for sinners. Many souls go to hell, because no one is willing to help them with sacrifice.