One key point that Jesus stressed during his time on earth was that the Kingdom of God was imminent. He told his disciples that they would not enter all the cities of Israel before the end:

Mat 10:23 When they persecute you in this city, flee to another. For assuredly, I say to you, you will not have gone through the cities of Israel before the Son of Man comes.

The end of the world (the English understanding of the world “apocalypse”) was near. This teaching was systematic in Jesus’ message (Mat 16:28 / Mar 9:1 / Luk 9:27, Joh 21:23, Mar 13:30, Luk 13:7, etc). James, in an undated letter (John Robinson points out that the dating of James could be anywhere), imbibes this apocalypticism. Towards the end of Chapter 4, James starts an interesting paragraph on planning for the future:

Jas 4:13 Come now, you who say, “Today or tomorrow we will go to such and such a city, spend a year there, buy and sell, and make a profit”;
Jas 4:14 whereas you do not know what will happen tomorrow. For what is your life? It is even a vapor that appears for a little time and then vanishes away.
Jas 4:15 Instead you ought to say, “If the Lord wills, we shall live and do this or that.”
Jas 4:16 But now you boast in your arrogance. All such boasting is evil.

The traditional Christian understanding of this passage is that people should just trust God. They engage in mysticism and claim that this passage means that we should just allow God to guide our lives rather than to plan our own. But this is not what the text is trying to communicate. James stresses death, mortality, God’s allowance of continued daily operations. And interestingly, James calls boasting “evil”. How many times have people heard a Christian boast about an upcoming trip to Europe? Are they evil?

But James is not condemning all future planning. Instead, James was upset that some people were planning their lives as if there would be no sudden end. He reminds people that death can come at any moment. James then links this thought to “the Lord’s will”. This is reminiscent of Peter’s explanation why the apocalypse has not yet occurred:

2Pe 3:9 The Lord is not slack concerning His promise, as some count slackness, but is longsuffering toward us, not willing that any should perish but that all should come to repentance.

The apocalypse is delayed by God’s will; it is because of this that James calls people who plan for the future “evil”. When James is talking about saying “if the Lord wills”, it is not saying “if God likes our plan better than other plans”. Instead it is saying “if God doesn’t decide to stop our plans short with some sort of apocalypse”. To emphasize his point, James continues:

Jas 5:1 Come now, you rich, weep and howl for your miseries that are coming upon you!
…
Jas 5:3 Your gold and silver are corroded, and their corrosion will be a witness against you and will eat your flesh like fire…
Jas 5:7 Therefore be patient, brethren, until the coming of the Lord. See how the farmer waits for the precious fruit of the earth, waiting patiently for it until it receives the early and latter rain.
Jas 5:8 You also be patient. Establish your hearts, for the coming of the Lord is at hand.
Jas 5:9 Do not grumble against one another, brethren, lest you be condemned. Behold, the Judge is standing at the door!

So what is going on here is that the evil people are condemned, people are told to wait patiently for the apocalypse (it is “at hand”), and the judge is standing at the door (in today’s world we might say “the guests have already arrived”). James is telling his readers the same message that Jesus preached: “The time is fulfilled, and the kingdom of God is at hand. Repent, and believe in the gospel.” (Mar 1:15)