From the Help Section in Excel:
Fill in a series of numbers, dates, or other items
1 Select the first cell in the range you want to fill, and enter the starting value for the series.
To increment the series by a specified amount, select the next cell in the range and enter the next item in the series. The difference between the two starting items determines the amount by which the series is incremented.
2 Select the cell or cells that contain the starting values.
3 Drag the fill handle over the range you want to fill.
To fill in increasing order, drag down or to the right.
To fill in decreasing order, drag up or to the left.
Tip To specify the type of series, hold down the right mouse button as you drag the fill handle over the range. Release the mouse button, and then click the appropriate command on the shortcut menu. For example, if the starting value is the date JAN-96, click Fill Months for the series FEB-96, MAR-96, and so on; or click Fill Years for the series JAN-96, JAN-97, and so on.
Hope that helps