theres no such thing as bad light, only the wrong subject.

Overcast days are perfect for macro work, discover the alien world which surrounds you every day!!!

Really?

I thought for macro you need good bright lighting, to do with the small focus area?

bright is nice, but even is critical.

Overcast days eliminate shaddows and reduce dynamic range. Makes normal photography flat and uninteresting, but makes macro much easier. You do have to be a bit careful for camera shake, but a modern SLR with image stabilisation makes this nice and easy. In the absence of image stabilisation, a tripod also works.