By default, the Date module will will append the "(All Day)" tag to any CCK Date field for which the time (hours/minutes) input has been left empty. This is pretty annoying. There are many ways to work around this, but the simplest by far is to simple override the date_all_day_label() function in your theme's template.php file.
You will need to replace "grasmash" with the name of your theme.
If you'd like to improve the date management process for your users, I'd also recommend installing the Date Tweaks module. It will, among other things, will add an "All Day" checkbox to your Date fields.