A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
In A2 I have entered the start date. If you are in USA you would type 6/30.
In B2 I typed =A2+21, and pressed Enter key
I click on B2 and observed the fill handle (samml sold square in lower right corner); when the mouse if over this, it changes to an + sign. Hold down left mouse button and drag to the right
You will need to widen the cell to see the date: double click on the line between the two column letters to automatically adjust the column width.
On row 3 I show that these are all Tuesdays
Formula in A3 is =A2 and then I gave it a custom format of ddd
best wishes - come back if more details needed