59 The series of catastrophes and the piety of Job is displayed now in comprehensive terms. Everything that can go wrong goes wrong, and yet Job, the pious servant of Yahweh, continues to worship him in the midst of the rubble. This section, and the next, will lay the foundation for the great dialogues in the book.
60 The to Job clarifies that it was the first day of the week. The fact that it was in the house of the firstborn is the reason.
61 Heb “his”; the referent (Job) has been specified in the translation for clarity.

In other versions

Now there was a day when his sons and daughters were eating and drinking wine in their oldest brother’s house,

One day when Job’s sons and daughters were feasting and drinking wine at the oldest brother’s house,

One day when Job’s sons and daughters were eating and drinking wine in their oldest brother’s house,

One day when Job’s sons and daughters were feasting at the oldest brother’s house,

And then there was one day when his sons and his daughters were eating and drinking wine in their firstborn brother’s house.

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

Public Domain

Job 1:13