Writable Calendar.

Found a few of these.

You just need to know what to search for.

Use Your Handwriting by Gee Whiz Stuff

https://appsto.re/us/zko7s.i

Palu - Shared Handwriting Calendar - by MetaMoJi Corporation

https://appsto.re/us/jMQlI.i

MemoCal lite version by plusryu

https://appsto.re/us/S3Wjz.i

MemoCal Plus by plusryu

https://appsto.re/us/bMD2A.i

PolyCalendar 2017 - Schedule and Handwriting - by SEUNG HYUN HAN

https://appsto.re/us/OLJ3eb.i

JotTheDate - Handwrite Calendar. by iStArtApp

https://appsto.re/us/df1f9.i

Check out the "MyScript Stylus" app. It's free, and it's awesome. The app is baisiclly a keyboard that allows you to use your pencil (or other stylus) rather than type. It translates to type in realtime, you can scribble out your mistakes and make other adjustments as you go. It doesn't add your handwriting-just translates it to text, but you can use it with any app.

There are certainly note taking apps that you can have calendar templates for. So, essentially, you're just writing on a calendar page just like a piece of paper. That means the data isn't available to other calendars or searchable or any of the other good things that you ordinarily get with a calendar on a device such as an iPad. Penultimate is one I can think of.

What did you end up using?