Hi ,
I am not having knowledge in office 365 but i am having some knowledge about exchange on-premise.
Note : I have given some information which is related to on-premise ,more or less on-premise and office 365 would be same.
1.If you wanted to know who has deleted that events in the calendar you can use the mailbox audit logs for tracing.But we need to manually enable the mailbox auditing and it will not be enabled by default.
2.In case if you wanted to have deleted items from the particular mailbox for a certain period .You can make use of the feature called In-Placehold.
Thanks & Regards S.Nithyanandham