[Avodah] The Cup for the Visitor: What lies behind the Kos Shel Eliyahu?

[Prof. Levine]

 From http://tinyurl.com/cle2efp
[An article on The Seforim blog by R' Eliezer Brodt. -micha]

One of the memorable parts of the seder night is during Shefoch Chamascha
when we open the door for Eliyahu Hanavi to come inside and drink from
the Kos Shel Eliyahu. Children all over the world look carefully to
see if there is less wine in the cup after he leaves, while many adults
accidently shake the table to make sure that there is less wine. What
are the sources of this custom? When do we pour the kos of wine and what
should we do with the leftover wine from the kos­drink it, spill it out,
or save it? In this article I hope to trace this custom to its earliest
known sources and to discuss some other aspects of the seder night
related to this topic.[1] I would like to point out that my intention
in this article is not to collect all the sources and reasons on these
specific topics but rather to focus on the earliest sources and how
these various minhagim came about.[2]

To begin with, it is worth pointing out that as far as we know today,
there is no mention of the concept of Kos Shel Eliyahu in all of
the literature that we have from the Geonim and Rishonim. Neither is
there mention of it in the Tur, Shulchan Orach, Rema, or other early
commentators on the Shulchan Orach.

See the above URL for more.

YL