London congestion charge on Saturdays?

I although I live on the edge of London I very rarely drive near the centre and do not understand this charge well. Today I think I have accidentally gone into the zone by mistake although I did not need to I took a wrong turn. As far as I can see it only applies Monday to Fridays but would be grateful if someone could confirm. How strict are they if you just go slightly into by mistake? Do they have hidden cameras or something?