‘Before the sun and the light and the moon are darkened, and the clouds return after the rain. In the day when the keepers of the house will tremble, and the strong men will bend themselves, and the grinders cease because they are few, and those who look out of the windows are darkened.'
They are to make the most of their lives before they become old and feeble, as described here. For the vivid description that now follows is of old age. The first sentence refers to a heavy enduring storm, when the day goes dark, the sun is hidden by the clouds, the light is simply semi-darkness, the moon also cannot be seen at night behind the clouds, and the rain fails to clear the sky of clouds, thus it is describing the time of life which brings with it the sense of waning strength, when light is hidden by clouds, and all is dull, and promise for the future is no more.
The second sentence describes picturesquely the failing faculties of old age. The keepers of the house which tremble are probably the shaking hands, the strong men which bend are the legs which can longer fully bear their weight, and the grinders which cease or are few are the teeth which have mainly decayed or fallen out, and are no longer any use for grinding, and what looks out of the windows are the eyes whose sight is dimmed. If anything is to be accomplished it must be before this assails a man.