CQRS - business validation rules

I'm creating a system using CQRS and Event Sourcing pattern (I hope so). I have to make a business decision dependent on statistics data stored by one read model and user setting data stored by different read model (both created by events in the past). What is a good place to put business logic rules that outcome of which depends on that data?

• Is it a command (can I fetch data stored in read models in command)?

• Other abstraction layer, like saga?