Here's a very quick non-formula way to get the same date each month.
Let's say you want to show the 20th of each month starting with
10/20/2005 beginning in cell A1.

1.) Enter 10/20/2005 in cell A1 and enter 11/20/2005 in cell A2.
2.) Highlight both cells.
3.) Drag the fill handle (the tiny black square in the bottom right
corner of the selected cells) down the desired number of rows and each
cell will now show the 20th of each month.

As with the formulas given, this will not work correctly if you use a
starting date that exceeds a valid date in other months (e.g., if you
start with 3/31/2005, it will not work because there is no 4/31/2005.)