I had an iui done february 8th. today is day 13 and i got a negative test. i was supposed to wait till tomorrow. can one day make a difference?

Usually not...: One day can make a difference, but usually does not. So, one can wait until 3 weeks after IUI before doing the first pregnancy test (there usually isn't a need to test sooner). Testing too soon can cause a false negative result and cause a woman to do a repeat test later. Testing at 3 weeks is accurate, whether the result is positive or negative. Patience is golden.