Adding existing sheets to folder in Numbers

Do you mean you have an August calendar in one document (based on the included template) and a September calendar in another separate document, and you would like to have them together in one document for convenience?

If so, you can do this.

In the September calendar click on the sheet "tab" at the top (named 'Monthly' on my machine) and choose 'Copy Sheet' from the dropdown menu that appears.

Then go to the August calendar and click on the "tab" there. Choose 'Paste Sheet' from the dropdown there.

You can then rename the sheet tabs from Monthly, Monthly-1, etc. to month names (or whatever) by double-clicking each one and editing it in place.

In the end you will have one consolidated document with each month on a separate sheet with the tabs named as you want. Once you are satisfied all is in order, you can delete the separate documents containing the monthly calendars.

Thereafter, a quick way to add a new month to your consolidated document: click an existing month tab and choose 'Duplicate' from the drop-down menu. Then edit the duplicate as desired. That way you don't need to start a new document based on the template each time.

SG

What do you mean by a 'common Numbers folder'. Numbers has no concept of 'folders'.

If you mean you have two separate documents, one for August and one for September, that you want to combine then you can select the tables from a sheet in one document and paste it into another (either the same sheet, or another sheet within the target document).

The only other thing I can think you might mean is to have one Numbers document that pulls in data from multiple other documents (e.g. a '2025' document that pulls in data from separate monthly documents). This isn't possible since Numbers does not support linking between documents. All data must be within the same document. However, if you consolidated each of the Month documents into one (by copy/pasting the tables into the yearly document), then you might be able to do something. It's not quite clear what it is you're aiming for.