There is no "best" update time, but there are a few hours that probably work best - it would depend on many things:
- around 6 a.m. GMT, because that's when people start to wake in Europe
- around 12 a.m. GMT, because that's when people start to wake in America
America and Europe are where most readers come from (more from America, I think)
You can also move both of those ~10 hours forward, because that's when most people finish school or work.
With that said, all those are for gaining new readers via latest updates, and there are not much people who search for stories there. As long as you update consistently, people who like your story will come.
As for days of the week, it's quite similar. There are people who read only on weekends or something, but as long as you get two chapters done in a week, they will come and read both whenever convenient for them.