You already know the answer, but for anyone else who wants to know the same thing:
You can generally
ADD new courses at any point until the end of Week 1.
You can generally
DROP courses at any point, with the following caveats:
* You will have to
pay for the course if you drop it after
Census date
* You will receive an
academic penalty if you drop it after
Drop deadline
You can find these dates on the
Key Dates calendar