At certain times of the year—especially around Samhain—you hear that “the Veil between the Worlds grows thin.”
This language speaks to something that many of us, I suspect, have ourselves experienced: those times when our Known World becomes an Otherness to us, often with such intensity—perhaps in response to that same Otherness within ourselves—that it seems we could step through and enter into the Interiority of things. In the lore this sense is frequently associated with the temporally and spatially liminal ( <Latin limen, “threshold”), the times and places of the In-Between.
As a quick web-search will demonstrate, the notion of the Veil Between Worlds comes to the Craft from Spiritualism; mediums are said to “part the Veil” to enable contact with the dead now in the Other World. Most likely Spiritualism derives the metaphor of the Veil (probably via Freemasonry) from the Veil of the Jerusalem temple, the curtain (Hebrew parókhet) that separated the “nave” of the temple from the Holy of Holies.