In general, admissions deadlines are the date materials need to be received by. However, some schools instead specify that the materials need to be postmarked by the deadline instead.
Usually, when it comes to standardized test scores, high school transcripts, letters of recommendation, and other materials not entirely under your control, admissions offices give a bit of leeway with the deadline. That said, try to get everything in as early as possible.
Many schools will also specify the latest SAT/ACT test dates they will accept. Make sure you pay attention to those requirements and take the test(s) in time.
It's better to look on the school's website first. They don't really appreciate receiving calls and emails about questions easily answered by checking the website.