I created a calendar and I have this issue with the navigation (at the above link only Sept - Nov)
If you look closely, the width of the menu cell and the big cell to the right changes when you navigate from September to October. It is a very small change that the average person would not notice, but I definitely notice.
This happens in both Mozilla and IE. (or at least IE 5.2 for Mac) The menu is from the List-o-matic website.
I'm not sure if this is what your talking about, but the only thing I saw wrong is the text "Calendar For 2006" is larger on the september section, than the "october" section. So when you click "october" link the menus move up some, because the "Calendar For 2006" text is a lot smaller. By the way, I noticed no change in width, just the placement of the menus move up some.
Could be because your using padding and the length of the names is slightly different - sept and nov the same but oct smaller, set up may and june and see if it still happens
I see the table width change, it is only slight. But there is a worse problem!. When going from Sept. through Qct. to Nov., there is a font size change in IE. Sept Calendar 2006 large, Oct. much smaller. Nov. back to large. Does not do this in FireFox.