If your bank Web site uses HTTPS, and you dutifully check that the server name in the URL is indeed the expected name, and you don't disregard warnings about unverified or expired certificates, then yes, it is safe.
If these conditions are not met, then no, it is not safe -- but it would not be safe from anywhere else either. Public WiFi is not special in that matter.
What may make public WiFi especially unsafe is that public WiFi occurs in public areas. Public areas are full of strangers. When you enter your bank password, you don't want weird people to spy on your keyboard or tablet screen. That kind of physical security is quite harder to achieve in a park or a restaurant, than in the privacy of your home.