I think it will be different for each person. The devil will want to attack us at our weakest point, and my weak point may be entirely different to yours. From experience, I can say that for me, it has been particularly when something has caused me to feel disheartened, sad, or upset, or when I have been particularly encouraged in the faith. We need always to heed Peter's words:I thought it might be when we are bereaved, upset, angry. That is when one might think evil spirits would attack , tempt and undermine us.
Yet it could be most likely we are most vulnerable when we have felt closest to our Lord God? When we have been baptized for instance? This was the point at which Jesus Christ Himself was tempted in the wilderness, just after his baptism.
It is interesting to note that Jesus was 'led up of the spirit into the wilderness to be tempted of the devil'. (Matthew 4, v 1).
So does the Holy Spirit lead us to the devil too, to be tested?
“Be sober, be vigilant; because your adversary the devil walks about like a roaring lion, seeking whom he may devour.” (1Pe 5:8 NKJV)