Check on the under side of leaves and flowers for tiny black specks. If they are there, you have spider mites. If the holes are very large (size of little finger tip) I would then guess slugs...
As In A Pickle says birds are probably the main culprits. Sparrows get very bored and do peck holes in brightly coloured flowers and especially love Forsythia and Pansies.
There is also a surge of baby snails about - another possibility.