Liminality; it is a term which many are unfamiliar with, to the extent that most spell checkers do not recognize it. For others, however, it is a concept which is of central importance. It has been my experience that anyone versed in the study of mythology is familiar, if not well versed, in discussions of liminality. My profession (well potential profession anyway) finds many of our activities occupying a very liminal period of time. From a GRP perspective, liminality is a central component of marking time and periods of significance.
Liminal, literally means threshold, the area between one area and another; the space in-between. A few people may be scratching their heads; a threshold? There is nothing significant about that, what amounts to little more than a few inches of space between where you are and where you are going. I'd argue though, that it is precisely because it is, quite literally "neither here, nor there" that it has occupied the imaginations of the folks who stop to think about just how powerful such a state really is. Borders have always been powerful, and much of how we understand, order and define space is based on the idea of what markers separate "here" from "there". Liminality; however, is not a way to create or define borders, because it is decidedly messier.
The ability to differing between two states can range from the simple to the complex. A dichotomy like inside and outside is far easier to define and understand, than say between life and death. Still, almost every culture has at some point defined what "life" and "death" mean. Since each of these aspects are so central to the human condition, the concept of liminality is of considerable importance when it comes to issues of death and dying. I mentioned earlier that my (eventual) profession is occupied with a liminal period, and that is between death and disposition. There is some sense, even among those who may not be religious, that the time between somatic death and final disposition (be it burial, entombment or cremation) is none the less an almost literal state of transition. I will have a lot more to say on this particular subject at a later date, but as I do not want to go off on a tangent; let me simply say that there is no other period in a persons life where I believe liminality is so apparent, than around a death.
In the lore, we encounter liminality all the time, for these are often where the important actions or events occur. In the Táin Bó Cúailnge, the majority of Cúchulain's combats occur at fords, areas of higher elevation creating shallow portions of a river or stream, which allow crossings. As rivers form natural barriers, the ford is a place in-between; a liminal area. The reason for the use of a ford as a battle ground is not simply for practical purposes, but because of the symbolism inherent in it as a liminal place. Similarly, the shore is also imbued with liminality, being the place between land and sea. The sea shore is often the place where the denizens of the otherworld interact with humans. In Immram Brain, we have Bran setting off for the otherworld in a coracle, and returning to this world and meeting folks on the shore. Of course as time flows differently, he appears to the inhabitants to be an otherwordly being himself and has become removed from the natural flow of time. Likewise the area's between land and sky, hilltops or mountains, also display the importance of liminality. While the strategic nature of building structures on higher elevations is quite clear, if one examines the tales, we find yet again the same sorts of symbolism associated. In one version of the arrival of the Tuatha De Danann, they arrive on clouds, causing a solar eclipse (more liminal symbolism) and "land" on a mountain top, before setting out to conquer Ireland.
Folklore too, is replete with activities which attest to the significance of liminal spaces or times. How often are door frames the place where charms are kept and windowsills where offerings are left?
Of all the holidays, Oíche Shamhna is the best example of a day which is in essence, dedicated to liminality. The day covers (or has been argued to be) the time between the new and old year. Further, the "veil" between this world and the otherworld, our realm and that of the gods, is thinnest. This had survived in folk beliefs through the belief that the veil between the living and the dead was also transversed with ease, and so one finds any number of divination customs in the texts. The idea that the ancestral spirits may also come to call, encouraged the setting of an extra place at a table, or leaving out of offerings for those wandering spirits. Conversely, as Lá Bealtaine in our world corresponds with Oíche Shamhna in the otherworld, the same symbolism (though to a lesser extent) is also observable in folks activities around that time as well.