Charles Spurgeon Commentary Job 1:12-13

"And Jehovah said unto Satan, Behold, all that he hath is in thy power; only upon himself put not forth thy hand. So Satan went forth from the presence of Jehovah. And it fell on a day when his sons and his daughters were eating and drinking wine in their eldest brother`s house," — Job 1:12-13 (ASV)

So Satan went forth from the presence of the LORD. And there was a day when his sons and his daughters were eating and drinking wine in their oldest brother's house:

That was a bad day for trouble to come. Satan selected that day because it was a joyful day, and therefore it would make the trials of Job the more startling. Moreover if Job could have had his choice, he would have preferred that his trouble should come when his sons and his daughters were praying, not when they were feasting.