How to generate Series of Dates
To generate a series of dates, do the following:
1. Enter the first date.
2. Drag the cell's fill handle while pressing the right mouse button (right-drag the cell's fill handle).
3. Release the mouse button and select an option from the shortcut menu:
Choose Series... to open the Series dialog box:
For example, if the Step value is 3, and in the Date unit section, the Month option is selected:
See also this tip in French: Comment créer une série de dates.