Some more information would help.
What is it in your Excel workbook that you want to use to trigger the e-mail message.
How will Excel know when it is the right day to trigger the message.
Will your workbook be open all day every day, or do you have to open it.
If you have a date in a cell and you want an e-mail if the date in that cell is today's date, you still need someway to trigger it. If cell A1 contains a date, a formula like this in cell A2
=IF(A1=TODAY(),"Send e-mail","Do nothing" will not change from "Do nothing" to "Send e-mail" unless the worksheet is re-calculated, so something must trigger the recalculation. The formula will recalculate when the workbook is opened, or if any other cell is changed, but if the workbook is just left open overnight, the formula will not change after midnight, unless some other cell is changed.
Information on using CDO e-mail in Excel can be found in two recent posts:
Reading through these may give you some ideas on what approach you want to take.