To hear the Banshee's wail means that death will soon pay a visit to your family or someone close by.
A Keening Wail in the Dark
The first night it is only heard by the person to whom death with soon visit. They begin as a series of moans about the rooms the target is present in. The moans are unmistakable, and the target is obvious, as they are the only one to hear them. This is thought to act as a warning, that death can be avoided, but with no idea where the tragedy will strike from, the warning is generally moot.
On the second night any argument about the Banshee’s visitation is put aside as the entire family can now hear the howling through the night. Now the family is certain that the following evening, the target will surely pass.
On the third and final night, the Banshee’s wailing turns into a full screech. This screech has the ability to break glass and pottery, and instils a total dread in those who hear it. Before the evening of the following day, the Banshee’s target will be dead. The Banshee will once again leave the family in peace, its job of foretelling a death complete... until death is once again due to visit the family.
That is one telling of the Banshee's wail. The Banshee, being a common figure of Irish folklore and mythology, has seen its legend travel along with several prominent Irish bloodlines. Once a figure only for several prominent Irish families - the O'Brien’s, the O'Connor’s, the O'Neill’s, the O'Leary’s, the O'Grady’s (the names do change with different sources.) - this spirit or sidhe from the Otherworld has followed along with marriages and unions to now occupy a much greater number of families, and is not limited to their traditional homeland.
Foreteller of Doom
It would seem that in more recent centuries the Banshee not only visits those of prominence soon to die, but even the commoners, albeit those with long distance relatives to 'royalty' of times gone by.
Tellings of a Banshee’s visitation vary from family to family, and although the shrieking is by far the most prominent, the Banshee has been seen to stand silently to the side of the bed of someone soon to die, or to be washing their clothes, or in the case of a knight/warrior, their armour.
There have been modern day reports of Banshee’s visiting families of Irish descent.
There are other cultures with similar figures present - Scottish, Welsh, Norse and American cultures all have similar legends.