Therefore put on the full armor of God, so that when the day of evil comes, you may be able to stand your ground, and after you have done everything, to stand.
There are good days and pretty good days, not bad, and then there are the bad days, hopefully not too many. Actually the passage quoted above may well be referring to the time in which we live, simply because we face the onslaught of the enemy. I know there are plenty of believers in Christ nowadays who chalk up the notion of the devil and demons to simply evil in general. When you consider everything, there’s something going on which defies an easy dismissal or explanation. Which is why in a world which denies truth, the word evil is still used, pointing, I think to something more beyond the relativism so prevalent. And to our personal lives, some days there seems to be an uncanny sense of an ongoing struggle and darkness at work, which casts a shadow on all that is good, so that we have trouble seeing the good. The Apostle Paul (click the link above) had no difficulty describing precisely what he believed is going on.
Scripture is rich with many places in which the person of faith is struggling, and all but left for dead in their mind, even with a sense of being abandoned by God. See especially the psalms to find plenty of that, and elsewhere.
As we’ve said before, and it is worth repeating, to be forewarned is to be forearmed. To know the problem, and what one faces is half the battle. The rest of it is to take up what God gives us in Christ and the gospel, and learn to stand in the midst of it all. Interpreting the spiritual warfare passage linked above as something for this present evil time when evil is so prevalent and active in the world is probably the best explanation, or sense of the passage. The thought the NIV gets across, that there may especially be a day that is evil, is also appropriate. But even with the NIV rendering, we are told to always be ready, so that everyday we are learning to live in the mighty strength of God, taking up the armor of God in and through Christ and the gospel.
Every day has a certain struggle to it, but bad days do come. And they go. And in the midst of it all, we need to remember the resources provided for us. And approach it as those who not only can face evil, but resist it. Knowing in the end it will someday be done away with forever. In the meantime we can be prepared for the day of evil in and through Jesus.