Problem with Excel fill handle

Whether Excel recognized month names depends on the region you have set in Windows Regional settings.

To find out which the month names are, enter a date like 1-1-2025 in a cell and drag this cell down using the RIGHT mouse button. Drag down 12 cells and when you release the mouse you can choose Fill months. Now format these cells to show month names in text, like mmm to show three letter abbreviations or mmmm to show the months in full. Your months must be spelled precisely like this.

If Excel shows a different spelling (language) of month names, you can either go change it in the Windows settings, or you can add the list of month names you want it to use as a custom list. Write all twelve month names into a column, then open File, Options and locate the Custom lists button on the Advanced tab close to the bottom. There is a box there to select a range of cells.