In general, there are two admission stages in one academic year. It’s either in Fall (usually in September) of in Spring (Febuary or April, depending on school system). The different starting semester (or quarter) naturally results in different application deadines. Fall semester/quarter has an application deadline on 15 January. The date varies from school to school of course.
Preferably, send your application packet to the admission office 1-2 months earlier. For some school, it has no advantages whatsoever; however some schools apply early review of the applicants, and hence will have less open lot for later applicants. Check the school websites for details. If no information is available, write to the admission committee.
For international students, only limited schools will offer Spring admission. Most of the students will join starting in Fall. It is wise to check if Spring admission is an option for you, and if there is opening for new students.