John Gill

And the Lord appeared unto him in the plains of Mamre,.... That is, to Abraham; and very likely this appearance of God was quickly after the affair of the circumcision, to show his approbation of his ready obedience to his command; and at this time he was in the plains, or at the oaks of Mamre, the oaken grove there, as has been observed on Gen 13:18; and which seems to be the best rendering of the words, since in Gen 18:4; mention is made of a tree to sit and stand under; and Abraham might choose this place for his habitation, because of the shadiness of it, in those hot countries: and he sat in the tent door, in the heat of the day; partly to cool and refresh himself, and partly to observe if any passengers passed by, to invite them in; this being a time of day when such needed refreshment, and it was proper for them to lie by a while, and not proceed on their journey until it was cooler: or rather to or "near" the tent door, as Noldius (g), or before it, without or under the shade of the tree, after mentioned. (g) Ebr. Concord. Part. p. 13.