Now upon a certain day when his sons and daughters were eating and drinking wine in the house of their eldest brother,

And the day is, that his sons and his daughters are eating, and drinking wine, in the house of their brother, the first-born.

And there was a day when his sons and his daughters were eating and drinking wine in their oldest brother's house:

And there was a day when his sons and daughters were feasting in the house of their oldest brother,

And there was a day - That is, on the day on which the regular turn came for the banquet to be held in the house of the older brother; compare the notes at Job 1:4.
And drinking wine - This circumstance is omitted in Job 1:4. It shows that wine was regarded as an essential part of the banquet, and it was from its use that Job apprehended the unhappy results referred to in Job 1:5.

There was a day - The first day of the week, says the Targum. It no doubt refers to one of those birthday festivals mentioned before.

And there was a day,.... Which according to the Targum was the first day of the week, but this is not certain, nor material; nor can it be said whether it was the day following that, Satan had leave to do what he would with Job's substance, nor how long this was after that; for though Satan was no doubt eager upon it, and in haste to do mischief; yet besides its requiring some time to get the Sabeans and Chaldeans to march out of their own country into Job's, so he would contrive and fix upon the most proper time to answer his ends and purposes, which was
when his (Job's) sons and daughters were eating, and drinking wine in their eldest brother's house; it should rather be rendered, "in the house of their brother, the firstborn"; that is, of Job; for relates not to brethren, but to parents, as Gussetins observes (b): this was either the beginning of a new turn, or rotation of their feasting with each other, which might begin with the elder brother; or this was his birthday; see Job 1:4 and this was the day Satan pitched upon to bring all the following calamities and distresses upon Job; partly that they might fall with the greater weight upon him, and more sensibly affect him, coming upon him while his family was feasting; and while he was pleasing himself with the thoughts of having brought up his children to men's and women's estate, and of the affluent circumstances they were in; and of the unity, harmony, and love that subsisted amongst them, of which their present feasting to gether was a proof; and partly that these afflictions might the more look like the judgments of God upon him, just as the men of the old world were eating and drinking when the flood came and destroyed them all, Luke 17:27 and for the same reasons these were all brought upon him in one day, to crush him the more; and that it might be thought the hand of God was in it, in a way of wrath and vengeance, and so irritate him to curse him to his face, which was what Satan aimed at; see Isaiah 47:8.
(b) Ebr. Comment. p. 127.

Satan brought Job's troubles upon him on the day that his children began their course of feasting. The troubles all came upon Job at once; while one messenger of evil tidings was speaking, another followed. His dearest and most valuable possessions were his ten children; news is brought him that they are killed. They were taken away when he had most need of them to comfort him under other losses. In God only have we a help present at all times.

JOB, IN AFFLICTION, BLESSES GOD, &c. (Job 1:13-22)
wine--not specified in Job 1:4. The mirth inspired by the "wine" here contrasts the more sadly with the alarm which interrupted it.

13-15 And it came to pass one day, when his sons and his daughters were eating and drinking wine in the house of their eldest brother, that a messenger came to Job, and said, The oxen were ploughing, and the asses feeding beside them, when the Sabeans fell upon them, and carried them away, and smote the servants with the edge of the sword; and I only am escaped alone to tell thee.
The principal clause, היּום ויהי, in which the art. of היּום has no more reference to anything preceding than in Job 1:6, is immediately followed by an adverbial clause, which may be expressed by participles, Lat. filiis ejus filiabusque convivantibus. The details which follow are important. Job had celebrated the usual weekly worship early in the morning with his children, and knew that they were met together in the house of his eldest son, with whom the order of mutual entertainment came round again, when the messengers of misfortune began to break in upon him: it is therefore on the very day when, by reason of the sacrifice offered, he was quite sure of Jehovah's favour. The participial construction, the oxen were ploughing (vid., Ges. 134, 2, c), describes the condition which was disturbed by the calamity that befell them. The verb היוּ stands here because the clause is a principal one, not as Job 1:13, adverbial. על־ידי, properly "at hand," losing its radical meaning, signifies (as Judges 11:26) "close by." The interpretation "in their places," after Numbers 2:17, is untenable, as this signification of יד is only supported in the sing. שׁבא is construed as fem., since the name of the country is used as the name of the people. In Genesis three races of this name are mentioned: Cushite (Genesis 10:7), Joktanish (Genesis 10:28), and Abrahamic (Genesis 25:3). Here the nomadic portion of this mixed race in North Arabia from the Persian Gulf to Idumaea is intended. Luther, for the sake of clearness, translates here, and 1-Kings 10:1, Arabia. In ואמּלטה, the waw, as is seen from the Kametz, is waw convertens, and the paragogic ah, which otherwise indicates the cohortative, is either without significance, or simply adds intensity to the verbal idea: I have saved myself with great difficulty. For this common form of the 1 fut. consec., occurring four times in the Pentateuch, vid., Ges. 49, 2. The clause לך להגּיד is objective: in order that - so it was intended by the calamity - I might tell thee.