I downloaded one of the themes, and I don't think that was necessarily what caused it, but my calendar application is no longer on my phone. I think it's possible I accidentally deleted it.
Check your /system/app folder for the apk first to confirm its gone. If so, you can just extract the apk for it from your roms zip file in the same directory. The stock calendar (and one included in sc) is named TouchWizCalendar.apk. Copy to sd card and move with a root file explorer or adb push it there.
The dark revenge "rom" (not a theme) has no calendar last I checked. Could that be to blame? You can always use an after market calendar without much in the way of complications. I use Jorte.