Excel has a very handy tool that speeds all kinds of data entry called the autofill handle. With it, you can insert an entire row or column of dates, numbers, text or other items. The autofill handle analyses the data you are inputting, and automatically repeats it in the manner of what has already been entered. It will read a series of dates and continue it as far as you need to go in your worksheet.
Enter the first series of dates into your worksheet in the manner you wish to see them. For example, you can enter "January" in one cell and "February" in the next if you are entering the months of the year. For dates, "January 1, 2005" followed by "January 2, 2005" will create consecutive days. You can enter the dates in any format you wish.
Click and drag over the cells you have entered to highlight them.
Place your cursor in the lower right corner of the cells you have highlighted. Hover over the corner until a black plus sign (+) appears (not the white plus sign with a black outline). When it appears, click and drag the mouse in the direction you want to insert the data.
Keep in mind that the data series will be continued based on the direction you pull in. If you have March above April, pulling down will insert the months going forward (May, June, etc.) in the cells below; if you pull up, the months going backward (February, January) will be inserted in the cells above.
Experiment with the tool for more difficult date patterns. Inserting two months is enough for the months of the year; for more complex data entry of dates, you may need to insert three, four or more cells before you get the results you want. You can also leave spaces in the series, and the autofill handle will read them and incorporate them into the series.