Sample Save The Date Birthday Dinosaur Excel Question - Is There A Formula That Can Automatically Update Based On The Day Of The Week?

Excel question - is there a formula that can automatically update based on the day of the week? - sample save the date birthday dinosaur

http://i30.photobucket.com/albums/c326/Jennifer__B/SampleExcelWorksheet.jpg

Above link to view a sample sheet mockery. To fill the column I am looking for AJ5 - AJ26. I've got it as close as possible to keep that size, because this is not my document for change. Thus, on information from the sample, 13 to, I want AJ totals column 1 to 12 days. On 14 I would be the sum of columns 1 to 14 as AJ, etc. Is there a formula for what will be updated automatically without having to manually change the formula every day? A solution to this problem would save my day!

If it helps, I have upgraded to a cell in the document automatically, the current date. However, I wonder if trying to be a bond at the time on the wrong track?