I've found something odd going on on our own server that I can recreate on the Zimbra demo server (Live demos of Zimbra's open source email and group calendar software).
The issue is described exactly in this bug, which was fixed in 4.5.4:
Bug 17048 – Calendar Notifications shows Incorrect day for recurring appointments
However, I'm still seeing it on our 5.06 as well as on Zimbra's demo server, so think maybe it's been re-introduced somehow.
I've sent this into support, but they've still not verified it. I've recreated it from two different machines (Mac & Windows FF).
I've done exactly this to recreate:
1. Open the calendar
2. Double-click on a Monday (say, 9/8) to create a new appointment
3. Fill with a subject (say, test1) and click More Details
4. In the detailed set up window, change the date to a Friday, (e.g. 9/12)
5. Set the appointment to repeat every week
6. Add yourself as an attendee and save.
7. Examine the invitation email, and you should see something like:
Recurrence : Every Monday. No end date. Effective Sep 12, 2008.
Note that it reads every "Monday", while the actual appontments are on Friday.
Am I the only one who sees this?