I would say that you were probably a bit early to catch ovulation on the 20th but it really depends upon your cycle. If you want to try to get a feel for your cycle by measuring basal body temperature, using ovualtion test kits or saliva/mucus testing. We've tried all of the above and found that the test kits were easiest (we did decide it was more fun guessing n the end).
Sperm can survive for a while in the fallopian tubes so it is better to be a little early than a little late.