Actually, while we do not have a date, we have at least a reason: "It was the apparent will and power of Melkor to effect his designs quickly and masterfully that had first attracted Sauron to him." (MT VII, i). This would be in the physical beginning of Eä, when M. still had the "vast demiurgic lust for power and the achievement of his own will on a great scale." (ibid.)
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