The custom of filling a goblet of wine for the Prophet Eliyahu at the Seder night on Pesach is not mentioned in the Gemara nor in the Rishonim, the early medieval halachic authorities. Its earliest source is in the writings of the Acharonim, the authorities of the sixteenth century and later. Why is this so? 

This custom is an expression of the Jewish people’s belief in the coming of Mashiach and in the coming of Eliyahu, who will herald the imminent Redemption. The nearer we approach the time of the Redemption, the more keenly is this faithful anticipation felt in the heart of every Jew. This is why the above custom came to light and became widespread in recent generations, even though we do not find tangible evidence of it in earlier days.


Ibid., Vol. XXVII, p. 55