Timer Tick Event only to execute at specific time
Here is another alternative...

private void tmrMain_Tick(object sender, EventArgs e) 
{ 
    if (ShouldRunNow())
        PerformClick();
} 

private void PerformClick()
{
    if (pnlAssembly.Visible)
        cmdShed.PerformClick(); 

    else if (pnlShed.Visible) 
        cmdServices.PerformClick(); 

    else if (pnlWeb.Visible)
        cmdAssembly.PerformClick(); 
}

private bool ShouldRunNow()
{
    TimeSpan startTime = new TimeSpan(8, 0, 0); 
    TimeSpan endTime = new TimeSpan(18, 0, 0);
    DateTime now = DateTime.Now;

    // Only run Saturday and Sunday
    if (now.DayOfWeek != DayOfWeek.Saturday &amp;&amp; now.DayOfWeek != DayOfWeek.Sunday)
        return false;

    // Only run between the specified times.
    if (endTime == startTime )
        return true; 

    if (endTime &lt; startTime) 
        return now.TimeOfDay &lt;= endTime || now.TimeOfDay &gt;= startTime; 

    return now.TimeOfDay &gt;= startTime &amp;&amp; now.TimeOfDay &lt;= endTime; 
}


Couple notes:
I think this is what ANeves meant by "would also consider extracting to a method the logic of whether or not to do the actions" but not sure so I decided to show it.  By putting the entire decision tree in a method you have self documenting code (the "if (ShouldRunNow())" describes what you are trying to do without needing comments).  

Eliminating "else" statements makes your code more readable (IMHO)

I personnally think using brackets when there is only one statement makes your code less readable as well.  Of course, this is my own opinion and I even have people at my workplace that disagree with this.  I will let the readers decide :)

Since the ShouldRunNow method is private then it doesn't hurt to not pass in the parameters.  And putting them directly in the method will make them easier to find.  This is not a big deal either way but just another alternative.

Update
Changed code based on feedback in comments