By mentioning Christ's "[landing] in force," Lewis is referring to the Second Coming, when Christ returns to earth to set up His kingdom. (The Second Coming of Christ is often confused with the Rapture of the Church, but eschatologically speaking they are two different events.) Passages referring to this even include the following:
Jude 14, 15 (ESV): "It was also about these that Enoch, the seventh from Adam, prophesied, saying, 'Behold, the Lord comes with ten thousands of his holy ones, to execute judgment on all and to convict all the ungodly of all their deeds of ungodliness that they have committed in such an ungodly way, and of all the harsh things that ungodly sinners have spoken against him.'"
Revelation 1:7 (ESV):"Behold, he is coming with the clouds, and every eye will see him, even those who pierced him, and all tribes of the earth will wail on account of him. Even so. Amen."
When Lewis says, "We do not know when," he is probably referencing Christ's statement made in Matthew 24:36 (and other places), "But concerning that day and hour no one knows, not even the angels of heaven, nor the Son, but the Father only."