The answer to this question will vary depending on your state. I suggest contacting your state's department of education (or researching online), to find the most accurate answer to your question. For instance, if you live in California, here is the Department of Education website: http://www.cde.ca.gov/index.asp , and here is information on their website about kindergarten enrollment: http://www.cde.ca.gov/ci/gs/em/kinderfaq.asp
You could also contact the local school district for your area to learn more about their enrollment policies and procedures. If you are located in the U.S., you can research the district's contact information via SchoolFinder: http://www.education.com/schoolfinder/
I'm including below some additional resources you may also find helpful.