Thanks for the tip... I played a little more, and discovered it's only requiring login when you click on a sub-calendar's event. Note that my sub-calendar pages ARE unsecure... they don't require login either... but if you go directly to a sub-calendar and click on an event there, the details load fine (don't require login). It's only when you click on a sub event in a "master" calendar that it requires login.

This sounds like a bug to me... ?

Sorry! I think I've actually found it now... it's not the subcalendars that cause it... it's the "Event Detail New Page" check.

Here's a URL created when events details are set to display inside the module on the existing page (default behaviour, "event detail new page" NOT checked):
http://www2.tacf.org/Home/ModuleID/1243/ItemID/1497/mctl/EventDetails/

Here's a URL created when events details are set to display in a new page ("event detail new page" IS checked):
http://www2.tacf.org/Home/ctl/Details/Mid/1243/ItemID/752/?ContainerSrc=[G]Containers/TACF/container-third-right-plain

The only time I've ever seen that "/ctl/details" string is on admin only pages.

Hmmm. I wonder if we have a bug in our DNN install. We have a similar problem with Xmod, where new pages opened require login. As if the security isn't set right. But it is set right! We'll upgrade to 4.9.3 this week and see if that helps. There's nothing in any of the release notes about this.