Magento – Magento 2: Event name before place order

I'm looking for event before any order placed. Need to do validation on observer.

https://www.mageplaza.com/magento-2-module-development/magento-2-events.html

If there is Bank Transfer then before clicked on "Place Order", If there is Paypal then Before went to Paypal.

Before it redirects to any third party Online/Offline it should be called.

I'm here <site url>checkout/#payment