Calendar issue: change from 2020 to 2021

A coworker's husband created a google sheet to keep track of student data and attendance. The problem is, the attendance calendar was originally formatted for 2020. Now when I put in 2021, the dates align, for the most part, with the correct days of the week. However, some beginnings and endings of the month don't populate correctly. What is wrong with the formula and how can I fix it? (please see attachment)

Administrative Note:

Welcome to the forum.

We would very much like to help you with your query, however the thread title does not really convey what your request is about. Tell us what you are trying to do, not how you think it should be done.

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