within SharePoint 2013 and from the browser, How do I set the default custom page layout to a master template?
The actual Page you are talking about is
_layouts/15/start.aspx is because of some new features (caching) implemented in SP2013.
The master page used by the page
calendar.aspx can be reviewed by opening the Site Settings -> Look and Feel (Master Page)
If you want to change the master of only this page, then you can edit the page in SharePoint Designer and change the