In the traditional Jewish Seder meal, the meal marking the beginning of the commemoration of the Passover of the Israelites in Egypt, there is always one chair set aside and left empty around the table.
The most common understanding of this tradition is that it is prepared for Elijah, who would announce the Messiah. This seat signifies the willingness of those gathered to serve this most important guest.
Abraham is visited by a surprise and most important guest - the Lord coming to him in the visit of three angels. Despite not knowing who it was before him, Abraham rushes to provide hospitality - clean water and food for a what appears to be three travelers. Only at the end of their visit do the angels reveal themselves as God's messengers. Jesus also comes to visit, to the home of Martha and Mary, but Martha's dissatisfaction in serving is a sharp contrast to Abraham's eagerness to provide.
What a blessing it would be to dine with the angels, or with the Lord in our own home! As we come to the Lord's house each Sunday and receive him, may we also receive those who come to us and serve them joyfully.