Talking about Magic Thresholds / Samhain

The Celtic festival of Samhain (November 1) is a "Threshold" festival - the "real" world and the "unseen" world are said to be very close at this point / the veil between the worlds is thin on this day. In the mythology, this is a recurrent day where it easier to contact the spirits, or for spirits to cross over to our world.

I have been musing about the same concept in regards to Tolkien's legendarium and wanted to throw it out to the community to get your thoughts. There are several devices that seem to be region or time-sensitive in the same operational way as Samhain - for purposes of discussion, a Threshold item.

Firstly, the Ring is a Threshold item; it moves a mortal from the seen to the unseen world. It grows in power / malevolence as it it approaches the physical location in which it was made; e.g. there may be is a permanent (at least, as long as the Ring exists) rift between the seen and unseen at Sammath Naur (the Cracks of Doom - this may be related to the *how* the Rings work broadly - a forced rift between the seen and unseen worlds.

Possible defeater of this line of thought - in "Grey Havens", Frodo is ill and muttering "It is gone for ever and now all is dark and empty" (referencing the Ring) in *early* March, as opposed to the expected (at least, by this theory) date of March 25 (when Sauron fell / when the Frodo claimed the Ring / when the Ring was destroyed) - early March doesn't really correspond to either the fall of Sauron, nor to Shelob's sting (March 12th, 3019).

Another Threshold item would be the Witch-king's blade, which was slowly turning Frodo into a wraith / dragging him over to the other side before it was removed. Again, this blade creates an annual rift between seen and unseen - a torment of mind and body which reoccurs on October 6th, every year to Frodo.

I would hazard along this idea that there might be permanent Threshold - where the walls between the worlds are thin - places - Dol Goldur, Minas Morgal, the Barrows, etc. I would say that possibly some of the Dwarf-doors (or Dwarf moon-letters) that are keyed to dates (Midsummer) or *disturbingly specific* phases of the moon (or both) might also be considered Threshold items. The Phial of Galadriel might be another, as it exerts power against seen and unseen simultaneously whilst making the user of it speak high-Elven.

Anyone have other items that might qualify / thoughts on this theory?