"Blessed are you that weep now, for you shall laugh," Jesus says (Luke 6:21). That means not just that you shall laugh when the time comes, but that you can laugh a little even now in the midst of the weeping because you know that the time is coming. All appearances to the contrary notwithstanding, the ending will be a happy ending. That is what the laughter is about. It is the laughter of faith. It is the divine comedy.
In the meantime you weep, because if you have a heart to see it with, the world you see is in a thousand ways heartbreaking. Only the heartless can look at it unmoved, and that is presumably why Jesus says, "Woe to you that laugh now, for you shall mourn and weep," meaning a different sort of laughter altogether-the laughter of callousness, mockery, indifference (Luke 6:25). You can laugh like that only if you turn your back on the suffering and need of the world, and perhaps for you the time for weeping comes when you see the suffering and need too late to do anything about them, like the specters of the dead that Jacob Marley shows old Scrooge as they reach out their spectral hands to try to help the starving woman and her child, but are unable to do so now because they are only shadows.
The happiness of the happy ending-what makes the comedy so rich-is the suggestion that ultimately even the callous and indifferent will take part in it. The fact that Jesus says they too will weep and mourn before they're done seems to mean that they too will grow hearts at last, the hard way, and once that happens, the sky is the limit.