The Welsh Triads lists the Three Fortunate Concealments and the Three Unfortunate Disclosures (see also Bendigeidfran's head was one of the Three Fortunate concealments; what are the other two?):
Three Fortunate Concealments of the Island of Britain;
The Head of Bran the Blessed, son of Llyr, which was conceiled in the White Hill in London, with its face towards France. And as long as it was in the position in which it was put there, no Saxon Oppression would ever come to this Island;
The second Fortunate Concealment: the Dragons in Dinas Emrys, which llud son of Beli concealed;
And the third: the Bones of Gwerthefyr the Blessed, in the Chief Ports of this Island. And as long as they remained in that concealment, no Saxon Oppression would ever come to this Island.
And there were the Three Unfortunate Disclosures when these were disclosed.
And Gwrtheyrn the Thin disclosed the bones of Gwerthefyr the Blessed form the love of a woman: that was Ronnwen the pagan woman;
And it was he who disclosed the Dragons;
And Arthur disclosed the head of Bran the Blessed from the White Hill, because it did not seem right to him that this Island should be defended by the strenght of anyone, but by his own.
It is possible to give approximate dates for some of these figures (for example, Gwrtheyrn the Thin is believed to be Vortigern, and we can thus figure out roughly when he lived), but I can find no information about when specifically the Concealments and Disclosures were made.
Does any text describe when each is thought to have happened? Alternatively, I'd love to find out when each Disclosure happened relative to its respective Concealments.