Perpetual Calendar Formula

Trying to manipulate(reformat) an existing calendar template to better suit my purposes. In order to do so I'd like to better understand the perpetual calendar formula to recreate the calendar from scratch. In the template (attached), the formula in C4 in the "January" tab reads:

=IF(DAY(JanSun1)=1,JanSun1-6,JanSun1+1)

where JanSun1 appears to be a named function or cell, but I can't find it. It seems to be referring to the "CalendarYear" named cell in N2&3, but I'm not grasping the logic in the formula. Also having a hard time understanding how the calendars for the other months on other tabs automatically update as the year on the spinner is changed. Is this a macro or purely formula?

Student Calendar_Sun Start11.xlsx

Any insight that anyone could give in helping me recreate the calendar in a blank worksheet would be helpful. Thanks.

This was the missing link I was looking for! Now I can follow the formula logic and think I can figure the rest out from here. Otherwise I'll be back! Thanks for the help.