When was Elijah first included in the Passover celebration?

The Sages who compiled the Haggadah also included the custom of placing a cup of wine on the table for Elijah the prophet. The main idea is that Elijah is the harbinger of of the Redemption, he lets us know about it. This is especially relevant on the Seder night, the first night of the Passover (Pesach) festival, which is one of the most likely and suitable times for us to be redeemed. In order to strengthen our belief in this, we leave the cup of wine for him, corresponding to the fifth cup of wine at the Seder.

The four cups of wine that we customarily drink at Seder correspond to four expressions of Redemption, and there is a discussion in the Talmud about whether or not to drink the fifth cup. The fifth cup corresponds to the expression ‘V’ heveti’, ‘and I shall bring you’ (to the land of Israel and the Temple – theBeitHaMikdash,). The conclusion of the debate is that Elijah the prophet will tell us, and this is another reason that the cup is called ‘the cup of Elijah’, for we will know whether or not to drink it, only when he arrives.

There are those who are of the opinion that since the Passover  (Pesach) Seder is one of the most widely observed Mitzvot, as almost all Jewish individuals and families feel a strong connection to this festival of Redemption, Elijah the prophet is sent to all the Jewish homes to see the beauty and the Jewish identification that is stronger than anything.