insert into event values('11:23:45 am','Birthday','24-dec-2008','Party');
You have 2 strings in different formats ('11:23:45 am' and '24-dec-2008') that you are inserting into a column of type TIMESTAMP. Oracle therefore needs to convert the strings to timestamps for you via an implicit conversion, which it does with the default format mask for your session, which is typically 'DD-MON-RR'. '11:23:45 am' doesn't match the format mask, hence the error.
I would not recommend splitting the date and time into 2 separate columns, but if I did I would probably do this: