Are you expexting your af to come on the
20th of january????
If so, then you can take a test the
earliest on jan.16th - 4 days before
you're expected af. Buy a "first
response" test, they work the best for
early testing. I would buy a pack of 2
just in case the first test comes out a
bfn, and you still do not get your af on
the 20th, you could wait a couple of days
and then test again.
Good luck!!!!