Deleted calendar items and recurrence changes are kept in the Rubbish Bin for 30 days before they expire.
Open Rubbish Bin from the mobile menu, add Rubbish Bin to your desktop header from Settings → Navigation, or open Settings → General → Rubbish Bin.
Select Restore to put an item back, or Delete permanently when you are certain you no longer need the restore copy.
Removing a linked birthday from the Address Book uses the same recovery area. Its birthday/preparation family is kept together in the Rubbish Bin for 30 days; restoring it returns the calendar family as standalone calendar items unless you link it to an Address Book contact again.