Six at the beginning means:

When there is hoarfrost underfoot,
Solid ice is not far off.

Just as the light-giving power represents life, so the dark power, the shadowy, represents death. When the first hoarfrost comes in the autumn, the power of darkness and cold is just at its beginning. After these first warnings, signs of death will gradually multiply, until, in obedience to immutable laws, stark winter with its ice is here.
In life it is the same. After certain scarcely noticeable signs of decay have appeared, they go on increasing until final dissolution comes. But in life precautions can be taken by heeding the first signs of decay and checking them in time.