When was my observation performed?
When was the file created?
What version of data processing was used?

All of this information can be retrieved from the header of the level 1 or level 2 event file with dmkeypar:

unix% dmkeypar acisf01843N002_evt2.fits DATE-OBS echo+
2000-09-02T08:35:02

unix% dmkeypar acisf01843N002_evt2.fits DATE echo+
2007-06-06T15:03:09

unix% dmkeypar acisf01843N002_evt2.fits ASCDSVER echo+
7.6.10

The above commands give the observation date, the file creation date, and the software version, respectively. The Note on Processing Versions explains the version naming convention.

If the file was created with certain CIAO tools, e.g. acis_process_events, then the ASCDSVER will reflect the CIAO version used:

unix% dmkeypar acis_1843_reprocessed_evt2.fits ASCDSVER echo+
CIAO