calendars are too wide in some themes

Start by defining a width for the calendar cells. You can do so via the styling page as instructed during install. If that does not help, post a link to your site and have a look at #1348520: Write and include documentation that also deals with styling problems.

IE9 works fine... so which version? Note that IE6 is not supported by default. I never tried myself, but I think that making split days work in IE6 is a mission (nearly) impossible. (This is due to the way they are rendered.)

The width's are there, so it "should" work. You could try the

.cal table {
  table-layout:fixed;
}

to force the width's. Let me know the results, I might adapt the base CSS for it.

I'm testing new version 7.3.1 on local.
I had same problem that exposed above with IE9. It's OK with FF or Chrome.
I played a little with IE dev tools. It appears I was with compatibility display. When changing this value to IE9, it was OK. In fact it's OK for IE8 and IE9 and KO for IE7 and compatibility IE9.

After that, I discovered that in IE9 tools, if I switch OFF that compatibility, it's OK.

I don't know a lot about that compatibility, but I ask myself, perhaps some anonymous user will have that compatiblity mode ON and will see that ugly calendar.

I joined a print screen image.

OK, I had another look at it. Only IE7 does this different and subsequently IE8 and IE9 in compatibility mode. The problem had to do with a theme giving a width of 100% to tables, conflicting with the width per cell settings of this module.

I added table-layout: fixed and width: auto to calendar tables and define the width as defined for day cells and weeknote cells also for the cells in the header row. For the latter you will need to regenerate the custom CSS file, which will also be done when running update.php.

dev version will appear on the project page within hours.

I tested new dev version (14th of January), it's OK in all versions of IE and always for FF and Chrome.
Now it's a little better in IE7 than in other navigators.
Thank you for that useful module and for your fix.

What do you mean with better in IE7?

Well forget it, it's because of my personal theme (Artisteer).

But if I test again with Bartik and IE dev tools, for IE7 and IE9 compatibility, days of week are not visible.

Attached, print screen KO with borders, in my theme (IE9 compatibility and IE7)
Attached, print screen KO with days, in Bartik, (IE9 compatibility and IE7)

For me because with my theme it's OK in IE8 and IE9, it's acceptable.

Thanks for the feedback. Bartik loves white header cell text :(. I changed the style generation to also define color's for the th's and td's, not only at the table level. This should make it more robust.

Committed once more.

But in the end, this module assumes that you do some styling yourself. It won't be foolproof with every theme, nor can you define all styles you may want to via the UI. And the presence of calendars on pages is too visible to not make it "in harmony" with the rest of your page.

I tested new commit (15th of January)
With IE and Bartik : it's OK qith IE8 and IE9. No days of week in IE7. So it's opposite of 2 days ago.
But important now is IE8 and IE9 so it seems OK.
I think as you say it can't work well in all possibilities and in all themes.
It's the difficulty of Drupal. You must be good at css.

To edit css, it's OK if I copy availability_calendar.base.css in sites/all/themes/mytheme ?
Drupal will look in that directory? No need to add that css in mytheme.info?

Assuming that you installed the latest dev, did you run update.php or regenerate the styling file manually?

- Edit widget and view have its own settings.
- Diagonal colors indicates that "split days" option is enabled for the (edit) widget.
- If all 12 months show at once, the edit widget selected is "shows the calendar as a number of months".

Is it not the difference between edit and view? overlay means you are editing, not logged in means viewing.