When certain system actions are taken, information about the change is logged into the audit trail table. To view the Audit Trail, go to System | Audit Trail.
To view all system entries for the last 30 days, you can just click on the Submit Query button. The audit trail for that period will be shown as in the example below.
Additionally, you can filter for entries for other things like a specific calendar, or specific calendar event. Change the entry type to the desired value, and specify the text you want to search for.
Finally, on many edit screens you'll see a last updated field:
If you click on the Audit Trail link, a pop-up window will appear with the change history for that entry: