What do you mean by birthday cal?
It's a new thing in the calendar app on iOS4, it's a calendar just for the birthdays of the people in your address book.
(I'm going to have a look as soon as my phone finishes it's backup)
Btw: it's 'rid' not 'read'.
It's easy to switch it off. Click on the Calendars button at the top left, scroll down to the bottom and deselect "Birthday" from Other.