When was God's plan of redemption determined?
God's plan of redemption was determined before anything was ever created, within the eternal counsel of the Trinity. All three persons of the Godhead foreknew and agreed together that this was the only way to bring it to pass, as described by the determinate counsel.
Les teaches that this plan was not a literal conference where the three sat around a table and debated, but within the Trinity of the Godhead, whether it was a split second or whatever, all three persons agreed on how they would accomplish redemption. It was all foreknown before anything was created and consummated in the thought process of the Trinity. The determinate counsel refers to the coming together and thinking through by the Godhead, and they all agreed this was the only way to make it come to pass.