Let's take an example with the CRM app. 

In the CRM app, the default date field used to present records in the calendar view is the "Next activity deadline". (Understand here the date of the next activity in the chatter of the opportunity/lead)


How can we change that? 

By clicking on the studio button on the upper right when being on the CRM calendar view, you are able to select the date field that needs to be used.