I had this suddenly happen with my calendar as well. It appeared on my Google Chromebook and iPhone XR. I attempted to create a new calendar on my phone. It was the original template. For a moment, it worked on the Chromebook too. Then, clicked to another note and it too reverted to this Day of Week/Date of Week separation. The new monthly calendar is still showing up the original template on my phone.
Does anyone know how to fix this? It's very important I have the template working in its proper format.