What works here is this (assuming you are using the template for Numbers 3):
Add a column at the right by clicking the symbol by the arrow in this screenshot:
The dates in the new column will fill in automatically. Type SUNDAY in the header of the new column.
Then hide (don't delete) the original SUNDAY column on the left.
That's it!
The steps are similar for the little calendars upper right, except that to hide the original Sunday columns there you need to first unmerge the cells in the header of each of those tables (select the header and choose 'Unmerge Cells' from the Table menu). After unmerging, drag the formula in A1 over one cell to the right and then hide column A. And finally select the cells in the header and merge again.
A lot of the math for the calendar, if you're curious, is in hidden rows and columns in the table on the upper left that displays the date and year. Unless you are a lot better at calendar math than I am, best not to mess with it, though.🙂
SG