In dream symbolism, a snake often represents transformation or a threat you haven't fully named yet -- its shedding skin makes it a natural image for a self about to change shape. A calm snake can point to healing or hidden wisdom; one that strikes usually reflects a fear or conflict you've been avoiding rather than a literal danger.
The Moon hath a certain natural mutability, therefore she easily communicates to us the Influence of the other Stars. - William Lilly (1602.-1681.)