Agree with @rokshana. If you're a foreign grad, you need it (and CS) before you apply. Period. If you're a US grad, it depends on your specialty, your Step 1, and which programs you're applying to.
If you had a strong Step 1 score, in most cases you need CK at some point before February (when rank lists are due). If you had a weak Step 1, earlier is helpful if you can make a good showing of it. Some programs care more than others - most CA IM programs require Step 2 to rank for example - but it's not typically a requirement to interview for US grads.
No, because students aren’t seen by programs in general.
Are you a F/IMG? Ideally you should have taken it by this month
Are you a USMD? Not a deal breaker for interviews...though as @Raryn said, if you have a crappy step 1 score, then it could help you to have a CK score to help counter it.