Google has added a new trash feature to Google Calendar so you can now get back the events you've accidentally deleted.
It works for both individual and recurring calendar events that have been deleted, and for your own calendar as well as other calendars for which you have edit rights. As the image above shows, to access your calendar trash bin, click on the drop-down arrow next to the calendar. You'll be able to permanently delete items from the trash bin as well.
This is rolling out over the next few days for Google Calendar on the web.
Restore deleted events in Google Calendar web [Google Apps Updates]