It sounds like something is slowly draining your battery. It could be as simple as an interior light that stays on that you didn't notice.
If you have had any electrical components installed recently, this could also be the cause. ie, alarm, radio, NGS.
I would recommend that you have a certified technician diagnose this problem before your battery is so depleted that it has to be replaced.
〉 Answered on Jul 30th, 2007 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines