To add a row e.g. into weekdays box, select a worksheet row which is contained in this box (click on row number at left border), copy the row (richt-click with mouse on activated row and select 'Copy' from dropdown menu), and then paste a new row into same position (right-click on row number at left border again, and select 'Insert Copied Cells' from dropdown menu).

To delete a row, you select a worksheet row you want to delete, right-click on activated row, and select 'Delete' from dropdown menu.

When you add or delete a row/rows, you have manually edit white colored rectangle which surrounds your tool (its size doesn't change when you add or delete rows) To activate the rectangle, you have to click on any of it's borders. You can see e.g. left border, when you select columns A:B.

Data Validation List values (x or ✔) are defined from menu Data>Data Validation : Allow = List

When you activate from menu Formulas>Name Manager, you see 2 defined Names. The value for Name StartDate is read from cell with date for Monday of week below your planner header. The value for Name WeekStart returns error. Probably at some time someone tried to make the planner to work with any date from week entered (enter any date, calculate monday of same week as WeekStart), but dropped the idea, and Weekstart is now abundant. The name StartDte is used to calculate day numbers in weekdays box headers, and for determining text format in weekdays box body.

In weekdays box, day numbers are calculated adding values 0...6 to StartDate

In Weekdays box, several Conditional Formatting rules (menu Home>Conditional Formatting Manage Rules) are defined, which must format texts entered for current day as strikethrough, and colour Data validation selections green (✔) or red (x). Those rules are currently quite messed up - some rules are defined for ranges where is nothing to format, there are ranges where no needed rules are defined, etc.