Suggestions for Group Calendar Software with Emil Notification?

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I have a kindergarten client who wants a fairly sophisticated web-based calendar
system with opt-in, email notification,
etc.  I'm not sure resource scheduling is important, this is more an event-based
deal.  Synchronization would be nice
with as many platforms as possible.

I haven't found any open source software that would suffice.  Does anyone have a
cost-effective suggestion?

I'm looking for a download/installation system, not a hosted service.

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