The easiest way to do this is via Applescript. The commands you need are both single line. To make Adium go online the Applescript code is:
tell application "Adium" to go online
To make it go invisible, the Applescript code is:
tell application "Adium" to go invisible
Pretty simple. You can also tell Adium to go away
. Open the Applescript Editor App in your /Applications/Utilities directory and create two Applescript files; one has just the command to go online and the other the command to go invisible/away. Save them as .scpt files with appropriate names.
That was the easy part. The next question is how do you trigger the right script at the right time? You essentially need to create the following two scheduled tasks:
"run adium_invisible.scpt at 5pm everyday"
"run adium_online.scpt at 8am everyday"
The default way to do this, built into OS X, is to use the UNIX 'cron' daemon. It's documentation can be found here. If you find the cron interface intimidating, there is a GUI app you can use to set tasks up, called Cronnix.
You'll still need to understand the timing syntax that crontab files use though. Wikipedia can get you up speed on that. Inside your user crontab you want to create two scheduled tasks. They should look like this:
0 8 * * * osascript path/to/adium_online.scpt
and
0 17 * * * osascript path/to/adium_invisible.scpt
osascript
is an Apple specific command that lets the shell call and run an Applescript. You give it the path to the .scpt file as a paramater. Once you have the two cron tasks set up you should be good to go.
If editing the crontab file by hand, and not using Cronnix, don't copy and paste these verbatim, as tabs are required between the hours, minutes, days etc., not spaces as I have here.
Also, you could create cron tasks that use osascript and pass the Applescript directly like this:
osascript -e 'tell application "Adium" to go online'
However, storing the Applescript commands in a separate file gives you flexibility in the likely event that you need to change what commands get run later on. You can add to or remove from the .scpt files without needing to interfere with the crontab file.
Best Answer
While I am sure its not a complete list (the only people who would have that would be Apple), here is a list I found on many of the phrases Siri understands:
Adding Events
Changing events
Asking about events
More available at http://www.iphonehacks.com/2011/10/iphone-4s-features-exhaustive-list-of-phrases-siri-can-understand.html
I personally have found that many of these features are available through the UI, but so complex or have a confusing workflow that most people wouldn't deal with it (for example, think of how many taps it would take to set a meeting at a time somewhere and invite someone?). For example, in the linked question about every three days, I wonder if it set a repeat that way (which is not available in the UI), or created new independent events.
After seeing your updated question, related directly to recurring monthly events on a certain day-of-week, I could not reliably get this to work. Using a phrase like 'schedule _ for the first Thursday of every month' did prompt for a time, but then tried to create the even for every day. This may work on a month to month basis, as in 'schedule __ for the second Tuesday of April'. I did another test just now, and it actually created a repeating event, but nothing more than a repeating one on the 30th of every month (it said it was creating a new event starting today). This was using the same phrase, and gave obviously different results.
I am not sure if this was due to a limit of Siri, or the fact that the recurrence options of Calendar on iOS is limited. This feature is easy to do on iCloud.com or in iCal itself.