Days/Posts Change to Month

I run a magazine and like to post the magazines articles for the entire month, not a certain number of posts of certain number of days (as the number of days in each month is different).

Is there an easy way to change the number of posts to show an option for a “Month” instead of the two defaults?

Thanks for any help.

Rusty
http://www.desertsaintsmagazine.com/dsmarticles