How can I automatically fill in a series of numbers or dates in Excel?

Enter a series of numbers or dates into a column:

Enter the first in the series manually and select that cell. Use the Fill Handle to drag the number or date to the end of the desired range.
Enter a series of incrementing numbers into a column:

Enter the first number manually. Select the cell and the range of cells that will be needed. Right-click and select Fill to bring up the Fill Series dialog box. Select Linear or Growth to create incremental values in the range.
Enter a series of months into a column:

Enter the the first month manually. Select the cell and the range of cells that will be needed. Right-click and select Fill to bring up the Fill Series dialog box. Select Month from the Type menu.