I'm not certain I understand the question fully but what I interpret it looks like your challenge is in you data sources. You have your data sources blended on date where you only have Student ID in one data source and Student Name in the other. Blending the data on date will only pull in students that completed Assessments on the same date which probably doesn't return the results you desire.
I think you will find your solution if you add Student ID to both data sources (Student Name also recommended) and join or blend your data on Student ID. Then you will have a more reliable data set and can still use your date values to create the calendar view you desire. You will then be able to pull in count of student names as well as pull in the details by student for both English and Math Assessments.
I hope this helps!
Thanks and best,