It really depends on when you think people are going to look at your application. The earliest you can submit your ERAS app is September 1st and you can submit that without necessarily having Step 2 scores in because you can retransmit your scores when you get new ones at any time once you submit ERAS. So it depends on what you're applying in and when it would be highest yield for programs to have scores. Obviously the latest you'd want them in for consideration by interview invite committees would be Nov 1 because that's when the dean's letter goes out. If you're applying in a very competitive field and you think it will really help your application you could consider having them ready by September 1st though. For my home program our transcripts weren't even uploaded into ERAS until mid-October at the earliest so you could also base your target date on when your school uploads transcripts.
As for when to take the exam, for CS you'll need to look up the score release dates, pick one you like, and then take the exam in the time frame for that score release date. For CK it typically takes up to 4 weeks for scores though around August sometimes there's been a delay in score reporting until early October so something to keep in mind if you're aiming for a Sept 1 score availability.
