alanb1 and Zbase4 the PSAT are administered throughout the country on 2 days. Tomorrow is the main day.
In terms of college admission... it counts for nothing. But it does count not just for NMS like coreyants mentioned, but there are many other programs that look into your PSAT score that might contact you about joining. For example, if you do well in the math portion, you might be invited to a math seminar over the summer at a university, which could lead to you getting an upper hand by saying that you attended at seminar over the summer at XYZ school.