The note in my study Bible says the "opportune time" comes at 22.3-6, when the devil enters Judas. This seems odd to me, as he does no work of temptation, only that of turning Jesus over to the authorities. The devil had to know that a dead Jesus was bad news for him.
So then maybe the "opportune time" comes in the Garden of Gethsemane. As Jesus' two wills battle against one another, "take this cup... but your will be done" the devil must have been right there hoping against hope that the human will would win.
I'm inclined to think differently, however. I've argued elsewhere that the devil was there tempting Jesus as he entered the Temple for the first time during what would become the week of his Passion. I think there was a pause while Jesus looked around because he pondered for a moment about whether or not he could really do it all by force.
My boss, however, expands the "opportune time" even further. His suggestion, that I am apt to agree with, is that the devil is right back in the next story, and the one after that, and the one after that. There are temptations all through Jesus' ministry; to give into the crowd at Nazareth, to let the unclean spirits tell everyone who he was, to let Peter fight his battle, and on and on.
If one takes this tack, then it makes a whole lot of sense to me that the "opportune time" didn't end on a cross at Cavalry, but continues to plague the Body of Christ to this day. The devil is here, tempting us with Biblical literalism, with the social gospel, with arguments about sex and money and women's ordination. The opportune time is now.
So then, what do we, the modern incarnation of the Body of Christ, do with this all? We stand on the power of Christ, remain filled with the Spirit, and confess him alone at all times and in all places. May you have the strength to withstand the devil at his most "opportune time." Amen.