Visit to the Imperial Palace of Tokyo

Hi
I want to know whether the outer city of the Tokyo Imperial Palace is opened to the public everyday except Mondays? And is the guided tour only to the inner city, and also operates everyday except Mondays? If I want to join the guided tour, is there any website I can book it?
Thanks!

Outer city? What is that?

The opening hours of the East Garden are explained here:
http://www.japan-guide.com/e/e3018.html

The operating days of the Imperial Palace tours are explained here:
http://sankan.kunaicho.go.jp/english/about/koukyo.html

The park area around the Nijubashi, Sakurada Gate outside of the inner moat, etc. are open 365 days a year 24 hours per day.

If I want to join the guided tour, is there any website I can book it?

Here you go:
http://sankan.kunaicho.go.jp/english/guide/koukyo.html

One more question, can I access the Chidorigafuchi from the Imperial palace? I mean walking from the imperial palace, is it far?

Yes, it is walkable. Here is a suggested route that leads from the Nijubashi to Chidorigafuchi through the East Gardens and Kinanomaru Park in about 45 minutes:
http://maps.google.com/maps?saddr=Unknown+road&daddr=35.682453,139.757...

Chidorigafuchi is the inner moat of the Edo(Tokyo) castle, and Edo castle is Tokyo Imperial Palace at present. So it's one part of the palace/castle.