Oracle SQL 11gR2 => find out all dates where "impdp" was performed
Every time a Data Pump job like that is started, the database notes this in the database alert log. If you have the alert log, you can find the entries there.

Other than that, the database does not store a history of the Data Pump jobs, unless you specifically instruct Data Pump to keep the metadata by adding the KEEP_MASTER=yes parameter to your job.