For baseball fans following along at home, here was the problem:
The calendar that Vicky was using had "Valid Date Range" properties set to look a the first record in her Case model. This record's Start and End dates were April 2 - so that was the only date in which new event records could be created.
The problem here is that in order to remove the valid date range you have to go to the XML. The model and field selectors cannot be "emptied" in the properties page. You have to go to the XML and remove the valid date range section from the code.